VARIOUS GENERAL CONSIDERATIONS CONNECTED WITH LIGHTHOUSES.

In the course of supplying the numerous wants of navigation, it will often be found necessary tocut off, on a given bearing, the beam proceeding from a Lighthouse, as a guide to the seaman to avoid some shoal, or as a hint to put about and seek the opposite side of a channel. This is attended with some little practical difficulty,especially in lights from reflectors arranged externally on a circle, because a certain portion of light, chiefly due to the divergence caused by the size of the flames, and partly from the effects of the diffraction or inflexion of the light, spreads faintly over a narrow sector between the light arc and the dark one.Masking Lights.It becomes necessary, of course, to make allowance for this penumbral arc by increasing the masked portion of the lantern; and, where a very sharp line of demarcation is required, a board is sometimes placed on the outside of the Lightroom, in such a position, and of such length, that while it does not enter the boundaries of the luminous sector, it prevents the more powerful part of the penumbral beam from reaching the observer’s eye. This effect is, of course, more conveniently produced, where the circumstances admit of its adoption, by distributing the reflectors round the concave side of the lantern, towards the land; but such an arrangement is inapplicable when the illuminated sector exceeds the dark one. I have found, by observation, that the sector intercepted between the azimuth on which the lantern is masked and that on which total darkness is produced to an observer, at moderate distances, may be estimated at not less than 3° for dioptric, and 7° for catoptric lights of the highest class.[79]

[79]The method which I adopted for determining those quantities, was to mask a certain portion of the lantern of a lighthouse subtending an horizontal sector of about 30° or 40°, and at night to fix, by actual observation, at the distance of 5 or 6 miles, two points on the coast between which the light so masked was obscured. The angle included between the lines joining those points and the centre of the lantern was then determined by triangulation next day, andhalf the differencebetween the observed angle (which is always thelesserof the two) and the computed subtense of the masked sector of the lantern, is, in each case, the amount of the allowance stated in the text.

Those quantities may therefore serve to guide the Lighthouse engineer to approximate more rapidly to his object, as he will generally be safe in increasing the dark sector, by one or other of the above constants, according to the kind of apparatus employed. I need not add, that in a matter of this kind, a final appeal to actual observation is, in all cases, indispensable.

A few words on the subject of double lights, naturally spring out of what has been said about the masking of lights.Double Lights.The termdouble lightsis properly and distinctly confined to lights on different levels, but notnecessarily(asleading-lightsare) in separate towers. The sole object of using double lights is fordistinctionfrom neighbouring lights; and they are unquestionably most effective in this respect, when they are placed in the same tower. In this point of view, therefore, I shall speak of them; and it is obvious that all that peculiarly belongs to them is, that the difference of level between them shall be sufficiently great to present the lights as separate objects to the eye of the seaman, when placed at the greatest distance at which it may be desirable that he should be able to recognise their characteristic appearance. In many cases it is not necessary (but it is certainly always desirable) that the lights should, from the first moment of their being seen, be known asdouble lights; but in others, it may well consist with safety, that two lights, which appear as a single light when first seen at the distance of 20 miles, shall at 15 or 10 miles distance be discovered to bedouble. Now we should at first be apt hastily to imagine, that all that is required to produce that effect, is to raise the one light above the other to such an extent, that the distance between them shall be somewhat more than aminimum visibileat the most distant point of observation, or, in other words, that the difference of the height of the lights should be such as to subtend to the eye at the point of observation, an angle greater than 13″·02, which is the subtense of aminimum visibileduring the day.[80]But the effect of irradiation, to which I have already alluded, tends to blend together the images of the lights long before their distance apart has become so low a fraction of the observer’sdistance from the Lighthouse, as to subtend so small an angle; and I have accordingly found by experiments, conducted under various circumstances, and at various distances, that repeated observations gave me 3′ 18″ as the mean of the subtenses calculated in reference to the distances at which the lights began to be blended intoone.

[80]This quantity is deduced from observations made by my friend MrJames Gardner, while engaged on the Ordnance Survey, and may be regarded as theextremelimit of visibility, under the most favourable circumstances as to the state of the atmosphere and also the contrast of colours. The observed object, also, was a pole, not arounddisc; and it is familiar to every one accustomed to view distant objects, thatvertical lengthis an important constituent in their visibility.

Adopting this as the smallest angle which the two lights should subtend at the observer’s eye, we may find the least vertical distance between them which will cause them to appear as separate objects by the following formula:

H = 2 Δ . tan θ

in which Δ is the observer’s distance in feet; θ, half the subtense, = 1′ 39″; and H the required height of the tower between the two lights in feet. The following Table gives the height in feet corresponding to the distance in nautic miles, from 1 to 20 inclusive: the heights, which are the bases ofsimilarisosceles triangles, increase, of course, in an arithmetical series:

Akin to the subject ofDouble Lights, is that ofLeading Lights.Leading Lights,the object of which is to indicate to the mariner a given line of direction by their being seenin one line. In most instances, this line of direction is used to point out the central part of a narrow channel; and the alternateopeningof the lights, on either side of theirconjunction, serves to indicate to the mariner (who ought to conjoin with his watching of the lights the observation of the elapsed time and also frequent soundings) the proper moment for changing his tack. In some places, the line of conjunction of the lights is placed nearer to one side of a channel than the other, according as the set of the tides, or the position of shoals, may seem to require. In other situations, this line only serves as across-bearingto shew the mariner his approach to some danger, or to indicate his having passed it, and thus to assure him of his entry on widersea-room. Similar considerations to those which determine the difference of elevation fordouble lightsregulate the choice of the distance between two leading lights; but the question is less narrow, and may be generally solved graphically by simply drawing the lines on an accurate chart of the locality. In some few situations, the configuration of the coast does not admit of a separation between the lights, sufficient to cause what is called asharp intersection; but, in most cases, there is room enough to place them so far apart, that but a few yards of deviation in the vessel’s course, from the exact line of the conjunction of the lightsin one, produces a distinct opening between them on the opposite side of that line. In order to insure the requisite sharpness of intersection, the distance between the lights, wherever attainable, should be not less thanone-sixthof the distance between the more seaward of the two Towers and that point at which the seaman begins to use the line of conjunction as his guide. I have only to add, that in situations where the land prevents a considerable separation between leading lights, they should be placed as nearly onone levelas is consistent with their being seen asverticallyseparated, so as in some measure to compensate for their horizontal nearness, by rendering their intersection more sharp and strikingthan it can be where the observer must draw from the upper light an imaginary perpendicular in his mind, and then estimate the separation of the lights by the sine of an angle, which decreases as the difference of their apparent elevations increases.

Distribution of Lights on a Coast.The considerations which enter into the choice of the position and character of the Lights on a line of coast, are either, on the one hand, so simple and self-evident as scarcely to admit of being stated in a general form, without becoming mere truisms; or are, on the other hand, so very numerous and often so complicated as scarcely to be susceptible of compression into any general laws. I shall not, therefore, do more than very briefly allude to a few of the chief considerations which should guide us in the selection of the sites and characteristic appearance of the Lighthouses to be placed on a line of coast. Perhaps those views may be most conveniently stated in the form of distinctpropositions:—

1. The most prominent points of a line of coast, or those first made onover-seavoyages, should be first lighted; and the most powerful lights should be adapted to them, so that they may be discovered by the mariner as long as possible before his reaching land.

2. So far as is consistent with a due attention to distinction, revolving lights of some description, which are necessarily more powerful than fixed lights, should be employed at the outposts on a line of coast.

3. Lights of precisely identical character and appearance should not, if possible, occur within a less distance than 100 miles of each other on the same line of coast, which is made by over-sea vessels.

4. In all cases, the distinction of colour should never be adopted except from absolute necessity.

5. Fixed lights and others of less power, may be more readily adopted in narrow seas, because therangeof the lights in such situations is generally less than that of open sea-lights.

6. In narrow seas also, the distance between lights of the same appearance may often be safely reduced within much lower limits than is desirable for the greater sea-lights; and there are many instances in which the distance separating lights of the same characterneed not exceed 50 miles, and there are peculiar cases in which even a much less separation between similar lights may be sufficient.

7. Lights intended to guard vessels from reefs, shoals, or other dangers, should in every case be placed, where practicable, to the seaward of the danger itself, as it is desirable that seamen be enabled to make the lights with confidence.

8. Views of economy in the first cost of a Lighthouse should never be permitted to interfere with placing it in the best possible position; and, when funds are deficient, it will generally be found that the wisest course is to delay the work until a sum shall have been obtained sufficient for the erection of the lighthouse on the best site.

9. The elevation of the lantern above the sea should not, if possible, for sea-lights, exceed 200 feet; and about 150 feet is sufficient, under almost any circumstances, to give the range which is required. Lights placed on high headlands are subject to be frequently wrapped in fog, and are often thereby rendered useless, at times when lights on a lower level might be perfectly efficient. But this rule must not, and indeed cannot, be strictly followed, especially on the British coast, where there are so many projecting cliffs, which, while they subject the lights placed on them to occasional obscuration by fog, would also entirely and permanently hide from view lights placed on the lower land adjoining them. In such cases, all that can be done is carefully to weigh all the circumstances of the locality, and choose that site for the lighthouse which seems to afford the greatest balance of advantage to navigation. As might be expected, in questions of this kind, the opinions of the most experienced persons are often very conflicting, according to the value which is set on the various elements which enter into the inquiry.

10. The best position for a sea-light ought rarely to be neglected for the sake of some neighbouring port, however important or influential; and the interests of navigation, as well as the true welfare of the port itself, will generally be much better served by placing the sea-lightwhere it ought to be, and adding, on a smallerscale, such subsidiary lights as the channel leading to the entrance of the port may require.

11. It may be held as a general maxim, that the fewer lights that can be employed in the illumination of a coast the better, not only on the score of economy, but also of real efficiency. Every light needlessly erected may, in certain circumstances, become a source of confusion to the mariner, and, in the event of another light being required in the neighbourhood, it becomes adeductionfrom the means of distinguishing it from the lights which existed previous to its establishment. By the needless erection of a new Lighthouse, therefore, we not only expend public treasure, but waste the means of distinction among the neighbouring lights.

12. Distinctions of lights, founded upon the minute estimation of intervals of time between flashes, and especially on the measurement of the duration of light and dark periods, are less satisfactory to the great majority of coasting seamen, and are more liable to derangement by atmospheric changes, than those distinctions which are founded on what may more properly be called thecharacteristic appearanceof the lights, in which the times for the recurrence of certain appearances differ so widely from each other as not to require for their detection any very minute observation in a stormy night. Thus, for example, flashing lights of five seconds interval, and revolving lights of half a minute, one minute, and two minutes, are much more characteristic than those which are distinguished from each other by intervals varying according to a slower series of 5″, 10″, 20″, 40″, &c.

13. Harbour and local lights, which have a circumscribed range, should generally be fixed instead of revolving; and may often, for the same reason, be safely distinguished by coloured media. In many cases also, where the purpose of guiding into a narrow channel is to be gained, the leading lights which are used, should, at the same time, be so arranged as to serve for a distinction from any neighbouring lights.

14. Floating lights, which are very expensive and more or less uncertain from their liability to drift from their moorings, as wellas defective in power, should never be employed to indicate a turning point in a navigation in any situation where the conjunction of lights on the shore can be applied at any reasonable expense.

Height of Lighthouse Tower, and its relation to range of Light.The spheroïdal form of the Earth requires that the height of a Lighthouse Tower should increase proportionally to the difference between the Earth’s radius and the secant of the angle intercepted between the normal to the spheroïd at the Lighthouse and the normal at the point of the light’s occultation from the view of a distant observer. The effect of atmospheric refraction, however, is too considerable to be neglected in estimating therangeof a light, or in computing the height of a Tower which is required to give to any light a given range; and we must, therefore, in accordance with the influence of this element, on the one handincreasethe range due to any given height, andvice versa reducethe height required for any given range, which a simple consideration of the form of the globe would assign. In considering this height, we may proceed asfollows:—

Fig. 92.Earth curvature illustrated

Fig. 92.

Referring to the accompanying figure (No. 92), in which S′dL′ is a segment of the ocean’s surface, O the centre of the earth, L′L a Lighthouse, and S the position of the mariner’s eye, we obtain the value of LL′ = H′, the height of the tower in feet by the formula,

H′ =2l²3(1.)

in whichl= the distance in English miles L′dat which the light would strike the ocean’s surface. We then reduce this value of H′ by the correction for mean refraction, which permits the light to be seen at a greater distance, and which =

2l²21,(2.)So as to get, H =2l²3-2l²21=4l²7(3.)

2l²21,(2.)

So as to get, H =2l²3-2l²21=4l²7(3.)

an expression which at once gives the height of the tower required,if the eye of the mariner were just on the surface of the water atd, where the tangent between his eye at S and the light at L would touch the sea. We must, therefore, in the first instance, find the distancedS =l′, which is the radius of the visible horizon due to the height SS′ =hof his eye above the water, and is, of course, at once obtained conversely by theexpression:—

l′=√7h2(4.)

Deducting this distance from SL, the whole effective range of the light, we have Ld=l, and operating with this value in the former equation,

H =4l²7

we find the height of the tower which answers the conditions of the case.[81]From the above data the following Table has been computed.

[81]In the above expressionslandl′are given in English miles, which in Scotland may be considered as bearing to nautical miles the ratio of 5280 to 6088. In order to convert a distance given in nautical miles to English miles, all that is needful is to add the log of the number of nautical miles to log 5280, and subtract log 6088.

If the distance at which a light of given height can be seen by a person on a given level be required, it is only needful to add together the two numbers in the column of lengths λ or λ′ (according as Nautical or English miles may be sought) corresponding to those in the column of heights H, which represent respectively the height of the observer’s eye and the height of the lantern above the sea. When the height required to render a light visible at a given distance is required, we must seek first for the number in λ or λ′ corresponding to the height of the observer’s eye, and deduct this from the whole proposed range of the light, and opposite the remainder in λ or λ′ seek for the corresponding number in H.

A considerable practical defect in all the lighthouse lanterns which I have ever seen, with the exception of those recently constructed for the Scotch Lighthouses, consists in the vertical direction of the astragals, which, of course, tend to intercept the whole or a great part of the light in the azimuth which they subtend.[82]The consideration of the improvement which I had effected in giving aDiagonal Lantern.diagonal direction to the joints of the fixed refractors, first led me (as stated atp. 266,ante), to adopt a diagonal arrangement of the framework which carries the cupola of zones and afterwards for the astragals of the lantern. Not only is thisdirectionof the astragals more advantageous for equalising the effect of the light; but the greater stiffness and strength which this arrangement gives to the frame-work of the lantern make it safe to use more slender bars and thus also absolutelylesslight is intercepted. The panes of glass at the same time become triangular, and are necessarily stronger than rectangular panes of equal surface. This form of lantern is extremely light and elegant, and is shewn, with detailed drawings of some of its principal parts, inPlate XXVI.To avoid the necessity of painting, which, in situations so exposed as those which lighthouses generally occupy, is attended with many inconveniences and no small risk, the framework of the lantern is now formed of gun-metal and the dome is of copper; so that a firstorder lantern of 12 feet diameter and 10 feet height of glass costs, when glazed, about L.1260. In order to give the lightkeepers free access to cleanse and wash the upper panes of the lantern (an operation which in snowy weather must sometimes be frequently repeated during the night), a narrow gangway, on which they may safely stand, is placed on the level of the top of the lower panes, and at the top of the second panes, rings are provided of which the lightkeepers may lay hold for security in stormy weather. A light trap-ladder is also attached to the outside of the lantern by means of which there is an easy access to the ventilator on the dome.

[82]I must also except the small pier light at Kirkcaldy, erected (I believe in 1836) by my friend MrEdward Sang.

Glazing of the Lantern.Great care is bestowed on the glazing of the lantern, in order that it may be quite impervious to water, even during the heaviest gales. When iron is used for the frames, they are carefully and frequently painted; but gun-metal, as just noticed, is now generally used in the Scotch Lighthouses. There is great risk of the glass plates being broken by the shaking of the lantern during high winds; and as much as possible to prevent this, various precautions are adopted. The arris of each plate is always carefully rounded by grinding; and grooves about ¹⁄₂ inch wide, capable of holding a good thickness of putty, are provided in the astragals for receiving the glass, which is ¹⁄₄ inch thick. Small pieces of lead or wood are inserted between the frames and the plates of glass against which they may press, and by which they are completely separated from the more unyielding material of which the lantern-frames are composed. Panes glazed in frames padded with cushions, and capable of being temporarily fixed in a few minutes, in the room of a broken plate, are kept ready for use in the Store-room. Those framed plates are calledstorm-panes, and have been found very useful on several occasions, when the glass has been shattered by large sea-birds coming against it in a stormy night, or by small stones violently driven against the lantern by the force of the wind.

Ventilation of the Lanterns.The ventilation of the lanterns forms a most important element in the preservation of a good and efficient light. An ill-ventilated lantern has its sides continually covered with the water of condensation, which is produced by the contact of the ascending currentof heated air; and the glass thus obscured obstructs the passage of the rays, and diminishes the power of the light. In the Northern Lighthouses, ventilators, capable of being opened and shut at pleasure, so as to admit from without a supply of air when required, are provided in the parapet-wall on which the lantern stands; the lantern roof also is surmounted by a cover which, while it closes the top of an open cylindric tube against the entrance of rain, and descends over it only so far as is needful for that purpose, still leaves an open air-space between it and the dome. This arrangement permits the current of heated air, which is continually flowing from the lantern through the cylindric tube, to pass between it and the outer cover, from which it finally escapes to the open air through the space between the cover and the dome. The door which communicates from the lightroom through the parapet to the balcony outside, is also made the means of ventilating the lightroom; and, for that purpose, it is provided with a sliding bolt at the bottom, which, being dropped into one or other of the holes cut in the balcony for its reception, serves to keep the door open at any angle that may be found necessary. A useful precaution was introduced by my predecessor, as Engineer to the Northern Lights Board, in order to prevent the too rapid condensation of heated air on the large internal surface of the lantern roof, which consists in having two domes with an air-space between them, as shewn in the enlarged diagrams inPlate XXVI.

An important improvement in the ventilation of Lighthouses was some years ago introduced by DrFaradayinto several of the Lighthouses belonging to the Trinity House, and has since been adopted in all the dioptric lights belonging to the Commissioners of Northern Lighthouses. After mentioning several proofs of extremely bad ventilation in Lighthouses, DrFaradaythus describes his apparatus:[83]

[83]Minutes of Institution of Civil Engineers, vol. i., p. 207.

“The ventilating pipe or chimney is a copper tube, 4 inches in diameter, not, however, in one length, but divided into three or four pieces; the lower end of each of these pieces for about 1¹⁄₂ inchis opened out into a conical form, about 5¹⁄₂ inches in diameter at the lowest part. When the chimney is put together, the upper end of the bottom piece is inserted about ¹⁄₂ inch into the cone of the next piece above, and fixed there by three ties or pins, so that the two pieces are firmly held together; but there is still plenty of air-way or entrance into the chimney between them. The same arrangement holds good with each succeeding piece. When the ventilating chimney is fixed in its place, it is adjusted so that the lamp-chimney enters about ¹⁄₂ inch into the lower cone, and the top of the ventilating chimney enters into the cowl or head of the lantern.

“With this arrangement, it is found that the action of the ventilating flue is to carry up every portion of the products of combustion into the cowl; none passes by the cone apertures out of the flue into the air of the lantern, but a portion of the air passes from the lantern by these apertures into the flue, and so the lantern itself is in some degree ventilated.

“The important use of these cone apertures is that when a sudden gust or eddy of wind strikes into the cowl of the lantern, it should not have any effect in disturbing or altering the flame. It is found that the wind may blow suddenly in at the cowl, and the effect never reaches the lamp. The upper, or the second, or the third, or even the fourth portion of the ventilating flue might be entirely closed, yet without altering the flame. The cone junctions in no way interfere with the tube in carrying up all the products of combustion; but if any downward current occurs, they dispose of the whole of it into the room without ever affecting the lamp. The ventilating flue is in fact a tube, which, as regards the lamp, can carry everythingupbut conveys nothingdown.”

The advantages of this arrangement, as applied to the Northern Lighthouses, were much less palpable than those which are described in the beginning of DrFaraday’spaper, because their ventilation was very good before its introduction; and the flame in particular was perfectly steady, being by no means subject toderangement from sudden gusts of wind from the roof in the manner noticed above.

Arrangements and internal management of a Lighthouse.All the Lighthouses in the district of the Commissioners are under the charge of at least two Lightkeepers, whose duties are to cleanse and prepare the apparatus for the night illumination, to mount guard singly after the light is exhibited, and to relieve each other at stated hours, fixed by the printed regulations and instructions, under which they act. The rule is, that no keeper on watch shall, under any circumstances, leave the Lightroom until relieved by his comrade; and, for the purpose of cutting off all pretext for the neglect of this universal law, the dwelling-houses are built close to the Light Tower, and means are provided for making signals directly from the Lightroom to the sleeping apartments below. These signals are communicated by air-tubes, through which, by means of a small piston, or a puff of wind from the mouth, calls can be exchanged between the keepers, enabling the man on guard in the Lightroom, at the end of the watch, or on any sudden emergence, to summon his comrade from below, who, on being thus called, answers by a counter-blast, to shew that the summons has been heard and will be obeyed. For the purpose of greater security, in such situations as the Bell Rock and the Skerryvore, four keepers are provided for one lightroom; one being always ashore on leave with his family, and the other three being at the Lighthouse, so that, in case of the illness of one lightkeeper, an efficient establishment of two keepers for watching the light may remain. At all the land-lighthouses also, an agreement is made with some steady person residing in the neighbourhood, who is instructed in the management of the light and cleansing of the apparatus, and comes under an obligation to be ready to do duty in the light-room when called upon, in the event of the sickness or absence of one of the lightkeepers. This person is called theoccasional keeper, and receives pay only while actually employed at the Lighthouse; but in order to keep him in the practice of the duty, he is required to serve in the lightroom for a fortnight annually in the month of January. The details of the lightkeeper’s duty may be seen by referring totheinstructionsalready alluded to, which will be found in the Appendix.

Each of the two lightkeepers has a house for himself and family, both being under a common roof, but entering by separate doors, as shewn inPlates XXVII.andXXVIII., which exhibit the buildings for the new Lighthouse at Ardnamurchan Point, on the coast of Argyllshire. The principal keeper’s house consists of six rooms, two of which are at the disposal of the visiting officers of the Board, whose duty in inspecting the Lighthouse, or superintending repairs, may call them to the station; and the assistant has four rooms, one of which is used as a barrack-room for the workmen, who, under the direction of the Foreman of the lightroom works, execute the annual repairs of the apparatus.

The early Lighthouses contained accommodation for the lightkeepers in the Tower itself; but the dust caused by the cleaning of those rooms in the Tower was found to be very injurious to the delicate apparatus and machinery in the lightroom. Unless, therefore, in situations such as Skerryvore, where it is unavoidable, the dwellings of the lightkeepers ought not to be placed in the Light Tower, but in an adjoining building.

Great care should be bestowed to produce the utmost cleanliness in everything connected with a Lighthouse, the optical apparatus of which is of such a nature as to suffer materially from the effect of dust in injuring its polish. For this purpose covered ash-pits are provided at all the dwelling-houses, in order that the dust of the fire-places may not be carried by the wind to the lightroom; and for similar reasons, iron floors are used for the lightrooms instead of stone, which is often liable to abrasion, and all the stonework near the lantern is regularly painted in oil.

If, in all that belongs to a lighthouse, the greatest cleanliness be desirable, it is in a still higher degree necessary in every part of the lightroom apparatus, without which the optical instruments and the machinery will neither last long nor work well.Cleansing of Apparatus.Every part of the apparatus, whether lenses or reflectors, should be carefullyfreed from dust before being either washed or burnished; and without such a precaution, the cleansing process would only serve to scratch them. For burnishing the reflectors, preparedrouge(tritoxide of iron) of the finest description, which should be in the state of an impalpable powder of a deep orange-red colour, is applied, by means of soft chamois skins, as occasion may require; but the great art of keeping reflectors clean consists in the daily, patient, and skilful application of manual labour in rubbing the surface of the instrument with a perfectly dry, soft, and clean skin, without rouge. The form of the hollow paraboloid is such, that some practice is necessary in order to acquire a free movement of the hand in rubbing reflectors; and its attainment forms one of the principal lessons in the course of the preliminary instruction, to which candidates for the situation of a light-keeper are subjected at the Bell Rock Lighthouse. For cleansing the lenses and glass mirrors, spirit of wine is used. Having washed the surface of the instrument with a linen cloth steeped in spirit of wine, it is carefully dried with a soft and dry linen rubber, and finally rubbed with a fine chamois skin, free from any dust which would injure the polish of the glass, as well as from grease. It is sometimes necessary to use a little fine rouge with a chamois skin, for restoring any deficiency of polish which may occur from time to time; but in a well-managed lighthouse this application will seldom, if ever, be required.

The machinery of all kinds, whether that of the mechanical lamp or the revolving apparatus, should also be kept scrupulously clean, and all the journals should be carefully oiled.

Mode of measuring the relative intensity and power of Lights.As I have had frequent occasion to speak of the comparative power of lights, it will not be out of place to present the reader with a few practical observations, chiefly drawn from the excellent work of M.Pecletto which I have so often referred, on the measurement of the intensity of lights by the method of shadows.

Fig. 93.Light beam

Fig. 93.

The intensity of light decreases as the observer recedes from the luminous body, in proportion to the square of his distance.Suppose abeam of lightto proceed from a radiant at F, and we shall have the rays which, of course, move in straight lines, gradually receding from each other, asb,b′,b″,b‴, andc,c′,c″,c‴, so that the section of the beam will increase with the distances Fb, and Fc; and the same number of rays, being thus spread over spaces continually increasing, will illuminate the surfaces with a less intensity. This decrease of intensity will, therefore, be in the inverse ratio of the extent of the transverse parallel sections of the luminous cones atbandc, which, we know, increase as the square of their distances from the apex of the cone at F. Hence we conclude, that theintensity of any section of a divergent beam of light decreases as the square of its distance from the radiant. This law furnishes us with a simple measure of the comparative intensity of lights. If we suppose two lights so placed that they may separately illuminate adjacent portions of a vertical screen of paper, we may, by repeatedly comparing the luminousness of those surfaces, and moving one of the lights farther from, or nearer to the screen, at length cause the separate portions of the paper to become equally luminous. This arrangement, however, has many practical difficulties, which I shall not wait to specify; but shall at once indicate a more simple and equally correct mode of obtaining the same result, by means of the shadows cast by the lights from an opaque rod, in a vertical position at O (fig. 94), placed between them, and a screen covered with white paper on which the shadows fall. It is obvious that the light at F would cause the object O to cast a shadow at SS, while the light at F′ would cast a shadow at S′S′. But while the shadow at S would still receive light from F′, S′ would receive light from F, so that those two shadows are, in fact, the only portions of the screen which are each illuminated only by one of the lights, while every other portionof its surface receives light from both the radiants at F and F′. If we suppose F to be the weaker light, we can bring it nearer the screen, until the shadow S′S′, shall become similar in appearance to the shadow SS; and we shall have the ratio of the intensity of the light at F to that of the light at F′, as (FS′)² is to (F′S)², which distances must be measured with the greatest exactness. Such is the mode commonly used in estimating the comparative intensities of two lights; but there are various precautions which are needful in order to prevent errors in comparing the deepness of the shadows, and to insure the greatest attainable accuracy in the estimate of the power of the lights, which I shall endeavour briefly to describe.

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