Fig. 94.Measuring light intensity
Fig. 94.
More accurate comparison of the intensity of Lights.The difficulties of estimating the deepness or sharpness of the shadow is very great, and many persons seem quite incapable of arriving at any right judgment in this matter. The same person also will discover such unaccountable variations in his decision after observations made at short intervals of time, as, one would think, can only arise from a sudden change of the intensity of one or both lights. M. Peclet, in hisTraité de l’éclairage, gives, as the result of his experience (and I can fully confirm his result by my own), that those differences depend less frequently on any real difficulty of estimating the deepness of the shadows, than on variations in the position of the observer, or rather in the angle at which he views the shadows, and that, consequently, in proportion to the distance between the two shadows, this source of error is increased. Any thing like a glossy texture of the surface of the screen, which then, of course, becomes a reflector, also tends to aggravate this evil. Thus, if the two lights which are to be compared be placed on a table, in such situations as to spread pretty far apart on the screen the shadows of a vertical rod placed between them; and if the shadow nearer to the observer seem to be a little deeper or sharper than the other, let the observer look at them from the other side of the table, and their difference will be reversed, and that which seemed the paler, will become the deeper. Again, if the difference between the two shadows be very greatwhen seen from the right side of the screen, it may happen that, on viewing them from the left hand, the difference may still be in favour of the same shadow, but in a much less degree.
“When I observed this effect,” says M. Peclet,[84]“I tried to view the shadows through a transparent screen, but I remarked the same variations. They were indeed even more sensible; for a variation in the distance of the eye of a few centimètres, made a prodigious change in the deepness of the shadows. I observed also that the shadow was much deeper when seen in the line of the light, and that in every other direction, it became paler in proportion as the eye receded from that direction.
[84]Traité de l’éclairage, p. 214.
[84]Traité de l’éclairage, p. 214.
“In all the cases which I have just described, the differences of the tints when the position is changed, increase in proportion as the shadows are farther separate; and they grow very minute when the shadows are almost touching each other.
Fig. 95.Studying shadows
Fig. 95.
“Let AB (fig. 95) be a white opaque surface,a, a luminous body, andm, a black opaque body, then the shadowb′cast on AB, will appear deeper when observed from P, than as seen from Q. This is a fact which may be easily verified, and the cause of which is easily conceived. In fact, the surface AB, although it disperses the light, must still reflect more of it, in the directions in which the regular reflection takes place; and hence the rays which are reflected round about the shadow, must have a greater intensity in the direction of P than in that of Q, and, consequently, the shadowb′must appear deeper from the point P than from Q.
Fig. 96.Studying shadows
Fig. 96.
“If we now place (fig. 96) two lights in front of the screen AB, at such distances that the two shadowsa′andb′should have equal intensities, it is evident that if the eye be placed at P, the shadowb′must appearmore intense than the shadowa′, and that the reverse will take place if the eye be at Q. But the difference which is then observed, arises not only from the difference in the brightness of the parts surrounding the shadows, but also from a difference in the intensity of the shadows themselves; for the shadowb′is illuminated byb, and radiates much more towards Q than towards P; and, on the contrary, the shadowa′, which is illuminated bya, radiates much more towards P than towards Q. We perceive also why thedifferencesof the tints increase with the separation of the two shadows, and why they become very small when the shadows touch each other; it is because, in proportion as the shadows are farther apart, each of them is illuminated more obliquely, and a greater quantity of light is radiated (by reflection) in the regular direction. When they touch each other, on the contrary, they are illuminated almost perpendicularly, and consequently the shadows radiate light almost equally on either side.
“Those anomalies of a like kind which are observed when the shadows are viewed through a translucent body, such as paper or linen, may be referred to a similar cause. We know, in fact, that, in looking through a translucent medium, we always, more or less, distinctly perceive the luminous body behind it, and, also, that there is a very large proportion of the rays which traverse the body, which stray but a little from the direction which they would follow if the substance were absolutely transparent. Consequently, the space which surrounds the shadow is more luminous in proportion as we come nearer to the direction of the shadow; and as the absolute intensity of the shadows diminishes as we come nearer to the direction of the rays which light them, those two effects concur to increase the intensity of that shadow to which the eye is nearer.
“As the dispersion by reflection is much more complete than by refraction, the variations of which we have just spoken are much greater with a transparent screen, through which the shadows are viewed, than with an opaque screen (from which they are reflected).
“This, then, is the mode of observing which has appeared to me the best, and by means of which we may obtain very great precision in measuring the intensity of two lights. I view, first, the two shadows in such a manner that both of them may be seen in succession from either side of the body which produces them, and at equal distances. For this purpose I use a good opera-glass. I alter the distance of the flames until in those two positions I perceive the differences (of the intensity in the shadows) to be in opposite directions. The distances of the lamps may then be considered as very nearly in the proper proportion for producing equal shadows, and to make them exactly so, the differences, which are observed on either side (of the centre line between them), should be equal; and, of course, the two shadows themselves, seen at one moment from either side of the opaque body, should be perfectly equal also.[85]These three observations, which mutually serve to verify or correct each other, will lead, with a little practice, to very great precision in the result. We may, also, by using a narrow screen, bring the shadows sufficiently near to touch each other; the variations of the tints then become very small by any change of our position, and we may, in this case, rest content with observing them from one point. To get rid of large penumbrae which are always an obstacle in forming a right estimate of the tints of the shadows, I place the opaque body very near the screen.
[85]I prefer to view the exterior portions of both shadows from the central line itself, in which case the opaque rod stands between them, because, in this manner, I obtain a more correct comparison by the direct contrast of the surfaces than by successive views of them, however quickly taken.
[85]I prefer to view the exterior portions of both shadows from the central line itself, in which case the opaque rod stands between them, because, in this manner, I obtain a more correct comparison by the direct contrast of the surfaces than by successive views of them, however quickly taken.
Fig. 97.Observation table
Fig. 97.
“When we wish to make a great many observations, it is very convenient to mark divisions on the table (which carries the lights), in order to read off, by means of them, the distance of the lamps from the shadows which they illuminate. By this means, each observation need not occupy more than two minutes. Igenerally use a table CC DD (fig. 97), about two mètres long (6 feet 6 inches), by 80 centimètres wide (2 feet 8 inches). At one end I place the screen AB, covered with white paper, dull (or not glazed), and kept in a vertical plane by two small pieces P and Q. Through the point M, the centre of the opaque body, I draw two lines Mfand Mg, equally inclined to the central linexy, whose extremitiesb′,a′are the axes of the two shadows. These lines must be inclined in such a manner that the distance of the shadows may be a little less than the diameter of the opaque body, or so that they may actually touch each other, according to the mode of observing which you wish to follow. These lines Mf, MgI divide into decimètres and centimètres, starting from the pointsa′,b′and over these lines I place the centres of the flames; the distance between the shadows remains always the same, whatever may be the distance of the lamps: to determine the distance of each lamp from the shadow which it illuminates, we ought, strictly speaking, to take the distance of the centre of the flamebfrom the pointa′; but as the distance from the pointbto the pointa′differs little from the distance between the pointsbandb′, we assume the latter for the former, without causing any sensible error. That distance may be obtained very conveniently by taking the half of the sum of the distances of the two extremitieszandz′of the diameter of the pedestal of the lamp. When the burner is not placed over the centre of the pedestal, we may suspend from it a small plummet,whose point will touch some division and indicate the distance between the centre of the burner and the shadow.
“When the lights are coloured, the shadows are coloured also, and it is then far more difficult to judge accurately of their intensity. They may in that case be much better seen from the pointx, as the black opaque body which is interposed between them renders the difference of colour less sensible to the eye.
“The opaque body M is a cylindric rod of iron, whose upper part is blackened in the flame of a lamp, in order to prevent the reflection which might interfere with thesharpness(netteté) of the shadows, and to make them more distinct when they are viewed from the point x.”[86]
[86]Those who feel a curiosity to look farther into this subject may consult Count Rumford’s elaborate paper in the Phil. Trans. for 1794, p. 67.
[86]Those who feel a curiosity to look farther into this subject may consult Count Rumford’s elaborate paper in the Phil. Trans. for 1794, p. 67.
I shall make a few trifling additions to M. Peclet’s clear description of his excellent mode of measuring the intensity of lights. It is, of course, presumed throughout, that the centres of the flames should be on one level; and I have found it most convenient to place the lamps on small carriages with rollers, which are guided by means of fine strips of wood nailed along the table in the directionsgM andfM, and carrying the divided scales of centimètres. This affords the means of making any slight change in the position of the lamps so easily, as entirely to avoid the disturbance of the flame which ensues from lifting the lamp and readjusting it in another position; and will, in practice, be found very convenient when many observations are to be made. I have already said that my own experience has satisfied me that, with the aid of a good opera-glass, the central observation of the two shadows, with the opaque rod between them, is by far the best, and conducts, at once, to a result which is confirmed by the observations of two assistants who watch the shadows at the same time on opposite sides of the table, and at equal distances from them. I have found it convenient in comparing lights, to cover the table with dull black linen cloth, and to surround it with curtains of the same material, hung from slender brackets, in such a manneras to leave space for the observer to move freely round the table within them. The curtains prevent reflection from the walls of the chamber in which the experiments may be conducted, and also lessen the disturbing effects of currents of air. When a comparison of theintensity, and not of theaggregate powerof two flames, is to be made, it is necessary to adopt the precaution of inclosing the lights in opaque boxes, with slits of equal area in each, placed on the same level, and so arranged, in reference to the flames, as to be directly opposite the brightest portion of each. After what has been said, it will be almost needless to add that thequotient of the square of the greater observed distance divided by the lesser, is the ratio of the illuminating power of the two flames. The most convenient mode of registering observations, and that which is generally practised, is in the form of a Table like thefollowing:—
As a standard lamp by which to test others, I believe few will be found superior to the best Carcel lamp, which has a clockwork movement, and whose flame continues to increase in power for about four hours after it is lighted; after which it maintains its state permanently, until the supply of oil fails. This fact was verified by M. Peclet with the greatest care. “I took,” says he, “two similar lamps. They were lighted at the same time, and their relative intensities were measured. One was then extinguished, without touching the wick, and its clockwork movement was stopped. One hour afterwards, I set the clockwork in motion and relighted the lamp, but without touching the wick. It was found in the same state as at the first comparison, and I measured its intensity in reference to the first. Those experimentsI repeated every hour, and these are the results which I obtained. The lamp which I call No. 1, is that which remained continually burning; No. 2, is that which was only lighted during the continuance of the (successive) observations.”
This curious scale of increase in power, seems to be solely due to a peculiarity of the manner in which the lamp, that derives its supply of oil by clockwork, becomes heated; and the effect may be described as follows: The heating of the wicks, the chimney, and the oil in this burner, as in that of all other lamps, tends to increase the light; but, in an ordinary lamp, acting by a constant pressure, thismaximumof heat is soon attained; whereas in the clockwork-lamp, into the burner of which the oil is thrown up by a pump, the whole of the oil in the cistern must reach its maximum temperature before thebesteffect of that lamp is produced. After this state has been reached, there is no disturbing influence at work, and the lamp burns steadily as long as the oil lasts.
I have myself tried what may naturally appear to be the most simple mode of obtaining an unvarying standard-light, by employing a gas-burner, supplied from a gasometer under a constant pressure; but I found it very difficult to obtain satisfactory proof of the constancy of the pressure; and in a large town, where there are many burners around one, their lighting or extinction is found toexercise a material influence in changing the condition of the flame. I must confess that I have always been disappointed in attempting to use a gas-flame as a standard of comparison.
There are various dangers on the shores of Britain, more especially at the entrance of the great estuaries of England and also in Ireland, whose position is such as to put them beyond the reach of regular lighthouses. Sand-banks which are too soft to sustain a solid structure, and have too deep water on them to admit of the erection of screw-pile lighthouses, are often the sites forFloating Lights.mooring light-vessels, to guide the mariner into the entrance of some estuary, or enable him to thread his way through the mazes ofgatsand channels, which, even during the daytime, baffle the mariner, who sees no natural object on the low sandy shores of the neighbouring coast to help him to guess at his true position. The first Light-vessel moored on the coast of Great Britain, was that at the Nore in 1734. There are now no fewer than 26 floating lights on the coast of England.
By the kindness of the Elder Brethren of the Corporation of Trinity House of Deptford Strond, I am enabled to give the following brief sketch of the nature and peculiarities of Floating Lights which was communicated to me by Mr Herbert, the secretary of theCorporation:—
“The annual expense of maintaining a Floating Light, including the wages and victualling of the crew, who are eleven in number, is, on an average, L.1000; and the first cost of such a vessel, fitted complete with lantern and lighting apparatus, anchors, cables, &c., is nearly L.5000. The lanterns are octagonal in form, 5 feet 6 inches in diameter; and, where fixed lights are exhibited, they are fitted with eight Argand lamps, each in the focus of a parabolic reflector of twelve inches diameter; but, in the revolving lights, four lamps and reflectors only are fitted. The greatest depth of water in which any light-vessel belonging to the Corporation of Trinity House of Deptford Strond at present rides, is about 40 fathoms (which is at the station of theSeven Stonesbetween the Scilly Islands and the coast of Cornwall).
“The Corporation’s light-vessels are moored with chain-cables of 1¹⁄₂ inch diameter, and a single mushroom anchor of 32 cwt., in which cases the chain-cables are 200 fathoms in length; some of the said vessels are moored tospan-groundmoorings, consisting of 100 fathoms of chain to each arm, and a mushroom anchor of similar weight at the end of each; a riding cable of 150 fathoms being in such cases attached to the centre ring of the ground chain. The tonnage and general dimensions of the light-vessel are given on the drawing of the lines.” (SeePlate XXIX.)
Still lower in the scale of “signs and marks of the sea,” areBeacons and Buoys.Beacons and Buoys, which are used to point out those dangers which, either owing to the difficulty and expense that would attend the placing of more efficient marks to serve by night as well as by day, are necessarily left without lights, or which, from the peculiarity of their position, in passages too intricate for navigation by night, are, in practice, considered to be sufficiently indicated by day-marks alone. Beacons, as being more permanent, are preferred to Buoys; but they are generally placed only on rocks or banks which are dry at some period of the tide. On rocks, in exposed situations, the kind of Beacon used is generally that of squared masonry, secured by numerous joggles (as shewn atPlate XXXII.); and in situations difficult of access, and in which works of uncompleted masonry could not be safely left during the winter season, an open framework of cast-iron pipes, firmly trussed and braced, and secured to the rock with stronglouis-bats, is preferred. The details of this framework are shewn atPlates XXX.andXXXI.A stone Beacon of the form and dimensions shewn inPlate XXXII., may be erected for about L.700, and the iron Beacon shewn atPlate XXX., for about L.640. In less exposed places, where the bottom is gravel or hard sand, a conical form of Beacon, composed of cast-iron plates, united with flanges and screws, with rust-joints between them, is sometimes used. A Beacon of this kind is shewn atPlate XXXII., which can be erected for about L.400.
Lastly, Buoys, which may be regarded as the least efficientkind of mark, and as bearing the same relation to a Beacon that a Floating-light does to a Lighthouse, are used to mark by daydangerswhich are always covered even at low water, and also to line out the fair-ways of channels. They are, for the most part, of one of the three forms shewn inPlate XXXIII., viz., theNun-buoy, in the form of a parabolic spindle, generally truncated at one end, so as to carry a mast or frame of cage-work, and loaded at the other end, so as to float in a vertical position; theCan-buoy, which is a conoid floating on its side; and, lastly, theCask-buoy, which is a short frustum of a spindle truncated at both ends, but almost exclusively used for carrying the warps of vessels riding at moorings. Those buoys are of various sizes and differ in cost. Mast-buoys, from 10 to 15 feet in length, cost from L.23, 15s. to L.48; and those of the Ribble and the Tay, which are 21 and 24 feet long, cost respectively L.105 and L.79; theCan-buoysare from 5 to 8 feet long, and cost from L.13, 13s. to L.20, 5s. Large buoys are often built onkneedframes resembling the timbers of vessels. The Cask-buoy is generally 6 feet long, and costs L.22, 15s. All these buoys are formed of strong oaken barrel-staves, well hooped with iron rings, and shielded with soft timber; and the nozzle-pieces at the small end of theNunandCanbuoys are generally solid quoins of oak, formed with aragletor groove to receive the ends of the staves. Much skill, on the part of the cooper, is required in heating and moulding the staves to the required form; and great care must be taken that they be of well-seasoned timber. Buoys are not caulked with oakum, but with dry flags closely compressed between the edges of the staves, which swell on being wet; and they are carefully proved bysteamingthem like barrels, to see if they be quite tight. Buoys are also formed of sheet-iron, in which case they are sometimes protected with fenders of timber; but they have been found more troublesome for transport, and, for most situations, are considered less convenient than those of timber.
In the beginning of 1845, I suggested the idea of rendering Beacons and Buoys useful during night, by coating them with somephosphorescent substance, or surmounting them with a globe of strong glass filled with such a preparation, whose combustion is very slow, and emits a dull whitish light and little heat. Some experiments were accordingly made by my suggestion; but I cannot add that any practically useful result has been obtained.
In laying down Beacons or Buoys, their position is fixed, as may be seen in theTablein the Appendix, either by the intersection of two lines drawn through two leading objects on the shore (the magnetic bearings of which are given for the sake of easy reference on the spot, in finding out the marks), or by means of the angles contained between lines drawn to various objects on the shore, which meet at the Beacon or Buoy from which they are measured by means of a sextant. In the latter case, the angles are always measured around the whole horizon, thus affording a check by the difference of their sum from 360°. The magnetic bearing of one of those lines is afterwards carefully ascertained, by means of the prismatic compass (if possible from one of the objects on shore, and if not, conversely from the Beacon or Buoy), so as to afford the means of translating the whole into magnetic bearings for the use of seamen. The buoys are moored, as shewn inPlate XXXIII., by means of chains and iron sinkers, with a sufficient allowance in the length of the chain to permit them torideeasily.