Fig. 76.
102. Leslie’s Hygrometer.—This instrument consists of a glass syphon tube, terminated with a bulb or ball at each end, turned outwards from each other, as in fig. 76. The tube is partly filled with concentrated sulphuric acid, tinged by carmine. One of the balls is covered smoothly with fine muslin, and is kept continually moistened with pure water, drawn from a vase placed near it by the capillary attraction of a few strands of clean cotton-wick. The descent of the coloured liquid in the other stem will mark the diminution of temperature caused by the evaporation of the water from the humid surface. The drier the ambient air is, the more rapidly will the evaporation go on; and the cold produced will be greater. When the air is nearly saturated with moisture, the evaporation goes on slowly; the cold produced is moderate, because the ball regains a large portion of its lost heat from surrounding bodies; and the degree of refrigeration of the ball is an index of the dryness of the air.
“Should the water become frozen on the ball, this hygrometer will still act; for evaporation goes on from the surface of ice in proportion to the dryness of the air. Leslie estimates, that when the ball is moist, air, at the temperature of the ball, will take up moisture equal to the sixteen-thousandth part of its weight, for each degree of his hygrometer; and as ice in melting requires one-seventh of the caloric consumed in converting water into vapour, when the ball is frozen, the hygrometer will sink more than when wet by 1° in 7°; and hence, in the frozen state, we must increase the value of the degrees one-seventh: so that each of them will correspond to an absorption of moisture equal to one-fourteen-thousandth part of the weight of the air.
“When this hygrometer stands at 15°, the air feels damp; from 30° to 40°, we reckon it dry; from 50° to 60°, very dry; and from 70° upwards, we should call it intensely dry. A room would feel uncomfortable, and would probably be unwholesome, if the instrument in it did not reach 30°.[8]In thick fogs it keeps almost at the beginning of the scale. In winter, in our climate, it ranges from 5° to15°; in summer often from 15° to 55°; and sometimes attains 80° or 90°. The greatest degree of dryness ever noticed by Leslie was at Paris, in the month of September, when the hygrometer indicated 120°.”—Professor Trail, in “Library of Useful Knowledge.”
In estimating the value of the indications of this hygrometer, it should be borne in mind that the scale adopted by Leslie wasmillesimal, that is to say, from the freezing to the boiling-point of water was divided into a thousand parts; ten millesimal degrees are therefore equal to one of the scale of Celsius.
103. DANIEL’S HYGROMETER.
Fig. 77.
This instrument was invented about the year 1820, by Professor Daniel, the distinguished author ofMeteorological Essays; and it entirely superseded all hygrometers depending upon the absorption of moisture. The form of the instrument is shown in fig. 77.
It consists of a glass tube, about one-eighth of an inch in diameter of bore, bent twice at right angles, and terminated, at each end, in a bulb about one inch and a quarter in diameter. In one limb of the tube is enclosed a delicate thermometer, which descends to the centre of the adjoining bulb, which is about three-parts filled with sulphuric ether. All the other parts of the tube are carefully freed from air, so that they are occupied by the vapour of the ether. This bulb is generally made of black glass; the other is transparent, but covered with a piece of fine muslin. The support for the tube has a thermometer attached, which shows the temperature of the external air. The tube can be removed from the stand, and the parts are made to pack, with a necessary phial of ether, in a small box, which can easily be got into the pocket.
How to use the Hygrometer.—This instrument gives the dew-point by direct observation, which must be made in the following manner:—Having fixed the tube upon the stand, with the bulbs vertically downward, the ether is all caused to flow into the lower ball by inclining the tube. The temperature of the air is noted by the exposed thermometer. Then some ether is poured, from a dropping tube fitting into the neck of the phial, upon the muslin-covered bulb. The rapid evaporation of this ether cools the bulb and causes condensation of the ethereal vapour in its interior. This gives rise to rapid evaporation of the ether in the lower bulb, whereby its temperature is greatly reduced. The air in the vicinity is deprived of its warmth by the cold bulb, and is soon cooled to the temperature at which it is perfectlysaturated with the vapour which it contains. Cooled ever so little below this temperature, some aqueous vapour will be condensed, and will form a dew upon the black-glass bulb. At the first indication of the deposit of dew the reading of the internal thermometer is taken: which is the dew-point.
This hygrometer has undeniable disadvantages. The surface upon which the dew condenses is small, and requires a peculiar direction of light in which to see it well. The observer, having his attention on the bulb and the thermometer, cannot always fix with precision the dew-point; and hence he is recommended to note the temperature at the appearance and at the disappearance of the dew, in order that the chance of error may be diminished. Without doubt, the necessarily long continuance of the observer near the instrument influences, to some extent, the observed temperatures; and the difficulty of not being always able to procure pure ether for the experiments is not the least of the drawbacks to the use of the instrument. Some of these disadvantages are obviated in Regnault’s hygrometer.
Fig. 78.
104. REGNAULT’S CONDENSER HYGROMETER
(Fig. 78) consists of a tube,C, made of silver, very thin, and perfectly polished; the tube is larger at one end than the other, the large part being 1·8 inches in depth, by 0·8 in diameter; this is fitted tightly to a brass stand,B, with a telescopic arrangement for adjusting when making an observation.
The tube,C, has a small lateral tubulure, to which is attached an India-rubber tube, with ivory mouth-piece; this tubulure entersCat right angles near the top, and traverses it to the bottom of the largest part.
A delicate thermometer,D, is inserted through a cork, or India-rubber washer, at the open end of the tube,C, the bulb of which descends to the centre of its largest part.
Gis an attached thermometer for taking the temperature of the air, andFis a bottle containing ether.
To use the Condenser Hygrometer, a sufficient quantity of ether is poured into the silver tube to cover the thermometer bulb: on allowing air to pass bubble by bubble through the ether, by breathing in the tube,E, an uniform temperature will be obtained; if the ether continues to be agitated, by breathing briskly through the tube a rapid reduction of temperature will be the result; at the moment the ether is cooled down to the dew-point temperature, the external surface of that portion of the silver tube containing ether will become covered with a coating of moisture, and the degree shown by the thermometer at that instant will be the temperature of the dew-point.
This form of hygrometer, for ascertaining by direct observation the dew-point, is so superior to Daniell’s, both from its being more certain in its indications andeconomical in use, that Messrs. Negretti and Zambra have been induced to modify it, and reduce its price to little more than that of a good Daniell’s Hygrometer.
Fig. 79.
105. Temperature of Evaporation.—When the air is not saturated with vapour, evaporation is going on with more or less activity, according as the temperature is high or low, rising or falling. Now vapour cannot be formed without an expenditure of heat; as we invariably find that the process of evaporation lowers the temperature of the liquid from which the vapour is produced, and, by communication, that of contiguous substances also. Thus the emigrant, crossing the line under the scorching influence of the vertical sun, wraps a wet towel round his can of water, swings it in the breeze, to evaporate the moisture of the towel, and obtains a glass of cool water. So also, European residents in India, during the hot season, spread out mats in their apartments, and keep them wet, in order that the evaporation may cool the air. This principle has been applied, for the purpose of ascertaining the hygrometric condition of the air, in the instrument known as Mason’s hygrometer, or psychrometer, which is now in general use, from its simplicity, accuracy, and ease of observing.
106. MASON’S HYGROMETER.
The Dry and Wet Bulb Hygrometer, or Psychrometer, known also as Mason’s hygrometer (fig. 79), consists of two parallel thermometers, as nearly identical as possible, mounted on a wooden bracket, one markeddry, the otherwet. The bulb of the wet thermometer is covered with thin muslin, and round the neck is twisted a conducting thread of lamp-wick, which passes into a vessel of water, placed at such a distance as to allow a length of conducting thread, of about three inches; the cup or glass is placed on one side, and a little beneath, so that the water within may not affect the reading of thedry bulb thermometer. In observing, the eye should be placed on a level with the top of the mercury in the tube, and the observer should refrain from breathing whilst taking an observation.
Thedrybulb thermometer indicates the temperature of the air itself; while the wet bulb, cooled by evaporation, shows a lower temperature according to the rapidity of evaporation.
To find the Dew-point.—From the readings of the two thermometers, the dew-point can be deduced by formulæ (that known as Apjohn’s is considered the most theoretically true), or from the valuable Hygrometric Tables by J. Glaisher, Esq., F.R.S.
For practical purposes in estimating the comparative humidity, the annexed table, which is a reduction from Mr. Glaisher’s elaborate work, will be sufficient; it will at least serve to assist in familiarising the inexperienced in the value of the psychrometer’s indications:—
The total quantity of aqueous vapour which at any temperature can be diffused in the air being represented by 100, the per-centage of vapour actually present will be found in the table opposite the temperature of the dry thermometer, and under the difference between the dry-bulb and wet-bulb temperatures. The degree of humidity for intermediate temperatures and differences to those given in the table can be easily estimated sufficiently accurately for most practical purposes.
The difference between the two thermometer readings taken from the reading of the wet bulb, gives the dew-point very nearly, when the air is at any temperature between freezing and 80°. This simple rule will be found serviceable to horticulturists, since it will enable them to estimate the chilling effect of dew or hoar-frost on tender plants.
Use as an Indicator of Weather.—In our climate, the usual difference between the thermometer readings,—in the open air, shaded from the sun, reflected heat, and currents of air,—ranges from one to twelve degrees. In hot and dry climates, as India and Australia, the range out of doors has been found as much as 30°, occasionally.
When the moisture is frozen, the bulb should be wetted afresh, and the reading taken just before it again freezes; but the observation then is of little value, and for general purposes need not be taken, as the air is known to be dry in frosty weather.
The muslin or cotton rag should be washed once or twice a week by pouring water over the bulb; and it should be replaced by a fresh piece at least once a month. Accuracy depends very much upon keeping the wet bulb clean, and nottoowet.
In connection with the barometer, this hygrometer is very useful, not only on land, but especially at sea, where other kinds of hygrometers cannot be practically used. A fall in the barometer is indicative of coming wind or rain: if the hygrometer shows increasing dampness by the difference of the readings becoming smaller,—rain may therefore be anticipated. On the contrary, if the hygrometer shows continuing or increasing dryness, a stronger wind is probable, without rain.
Domestic Uses.—Mason’s hygrometer is useful in regulating the moisture of the air of apartments; a difference in the thermometer readings of from 5° to 8° being considered healthy. Many complaints require that the temperature and humidity of the air which the invalid breathes should be carefully regulated. Hence it is a valuable household instrument. In a room, it should be placed away from the fire as much as possible, but not exposed to draughts of air.
Figs. 80 and 81 show cheap arrangements of the instrument for domestic purposes. Other arrangements are given to the instrument to make it suitable for exhibiting the hygrometrical state of the air in hot-houses, conservatories, malting-houses, warehouses, manufactories, &c.
Fig. 82 shows the instrument arranged on brass tripod stand, with folding legs and metal cover, to render it portable.
107. Self-Registering Hygrometer.—A maximum thermometer and a minimum thermometer, each fitted up as a wet-bulb thermometer, record the highest and lowest temperature of evaporation during the interval of observation. Negretti’s mercurial maximum, and an alcohol minimum, answer best.
108. Causes of Dew.—“The aqueous vapour of our atmosphere is a powerful radiant; but it is diffused through air which usually exceeds its own mass more than one hundred times. Not only, then, its own heat, but the heat of the large quantity of air which surrounds it, must be discharged by the vapour, before it can sink to its point of condensation. The retardation of chilling due to this cause enables good solid radiators, at the earth’s surface, to outstrip the vapour in their speed of refrigeration; and hence, upon these bodies, aqueous vapour may be condensed to liquid, or even congealed to hoar-frost, while at a few feet above the surface it still maintains its gaseous state.”[9]The amount of moisture so deposited will vary with different atmospheric conditions. If the sky be decidedly cloudy or misty, the heat radiated from the earth will be partly restored by counter-radiation from the visible vapour; the cooling of the earth’s surface will, therefore, take place slowly, and little dew will be deposited. On the other hand, if the air contain transparent vapour, and the sky appear clear, the counter-radiation will be less, the earth will cool rapidly, and the deposit of dew will be copious; provided the night be comparatively calm, for, when the wind blows, the circulating air supplies heat to the radiating substances, and prevents any considerable chilling.
The dew which falls in tropical countries greatly exceeds in abundance what we experience in our climate; because the air is there, from the great heat, capable ofsustaining a large amount of vapour in the transparent state, and the conditions most favourable for a maximum reduction of temperature by radiation are present. At those places, or upon those substances which cool the lowest and most readily, the dew falls most copiously.
Fig. 83.
109. Plan of Exposing Thermometers, &c.—Figure 83 is an illustration of a convenient slab for supporting thermometers in an exposed position attached to a stand (such asGlaisher’s, described in Chapter XVI.) for ordinary scientific observations. It has a projecting ledge,B, to carry off rain from the instruments, the slab,A, being erected vertically. The hygrometer is placed atE, with the vase of water atF. An alcohol minimum thermometer is represented atC, in the position most favourable to its certain action; and atDis shown one of Negretti & Zambra’s maximum thermometers, the position of which may be more nearly horizontal than there exhibited, although a slight depression of the bulb-end of the frame is desirable, but not necessary, as this thermometer can be used in any position.
INSTRUMENTS USED FOR MEASURING THE RAINFALL.
The instruments in use for measuring the quantity of rain which falls on a given spot are of very simple construction. Perhaps the simplest is:—
110. Howard’s Rain-Gauge.—It consists of a copper funnel, a stout glass or stone bottle, and a measuring glass. The bottle is to be placed upon the ground, with the funnel resting on its neck. A brass band or cylinder fixed upon the outer surface of the funnel envelops the neck of the bottle, and the pipe of the funnel extends nearly to the bottom of the bottle; so that loss by evaporation is avoided as much as possible. The receiving space of the funnel is formed by a brass ring, five inches in diameter, very accurately turned. The measuring vessel enables the observer to note the rainfall in inches, tenths, and hundredths of an inch.
Fig. 84.
111. Glaisher’s Rain-Gauge.—The rain-gauge designed by Mr. Glaisher, the well-known meteorologist, and used by most observers of the present day, is arranged for the reception of the water which falls upon its receiving surface only, and for the prevention of loss by evaporation. The rain is first collected in a funnel,B, (fig. 84,) the receiving surface of which is turned in a lathe. The conical surface of the funnel slopes to the pipe,E, at an angle of 60° from the horizontal receiving surface. The tube,E, is of small aperture, and is bent up, in order to retain the last few drops of rain, so that the only opening for the escape of vapour may be closed as long as possible. The funnel,B, fits upon the cylinder,A, tightly in the groove,D. A copper can is placed inside the cylinder,A, to receive the rain from the funnel. Once or twice a day, or after a shower, this can should be taken out, and the water measured in the glass measure,C, which is graduated to hundredths of an inch, according to the calculated quantity of water, determined by the area ofthe receiving space. In use, this gauge should be partly sunk in the ground, so that the top may be about five inches above it. Thus situated, there will be little or no evaporation from it during any month of the year; and the readings need not be taken daily, although desirable.
112. Rain-Gauge with Float.—In this construction the graduated glass measure is dispensed with. The cylinder of the gauge is made less in diameter than the funnel, and a hollow, very flattened spheroid of copper forming a float, and carrying a vertical graduated boxwood scale which moves through the orifice of the funnel, is placed in it. As the rain accumulates the float rises, and the amount of rain in the gauge is read upon the scale from the top of the gauge, a bar, having a hole at the centre for the passage of the scale, being fixed diametrically across the receiving space of the funnel. The gauge is provided at the bottom with a brass cock, by which the water may be allowed to flow out of it whenever necessary.
This form of gauge is not very suitable for the measurement of small quantities; but is admirably adapted for localities where the rainfall is excessive.
Fig. 85.
113. Rain-Gauge with Side-Tube.—This instrument, as represented in fig. 85, is a cylindrical vessel, mounted on a base shaped as a frustum of a cone. This base may be filled with sand or gravel to make the instrument stable, so that when placed upon a lawn or in a garden it may have an ornamental appearance. The funnel for collecting the rain is larger in diameter than the cylinder. Parallel to the cylinder, and communicating with the lowest part of the interior and extending to its top, is a graduated glass tube, open at both ends. The rain collected will rise as high in this tube as in the cylinder, and its amount can therefore be read off without any trouble. The gauge is emptied by the brass tap at the bottom of the cylinder.
114. Admiral FitzRoy’s Rain-Gauge.—A form of rain-gauge, very well adapted for expeditious observation at any time, has been designed by Admiral FitzRoy, and extensively employed by his observers. It is cylindrical in shape, with the funnel let into the top; and the rainfall is collected in an inner and much smaller cylinder, so that a small fall is represented by a considerable depth of water in the gauge. The amount of rain which has fallen is ascertained by a dipping tube, similar in principle to the dipping syphon used by gaugers for taking out specimens of wines or spirits from casks by simply removing the bung. A short, vertical, tubular opening provided with a cap, which is attached to the instrument by a chain that it may not be lost, is formed in the funnel. The measuring tube, which has a small hole at each end, should beplaced upright in the gauge; then the thumb should be pressed over the upper aperture, while the tube is lifted gently out, holding in the lower part a quantity of water representing the depth of the rain in the gauge, the upper edge of which is at the mark to be read off. The glass tube is graduated to inches and tenths; hundredths of an inch can be readily estimated by the eye. The marks are fixed by actual trial with a standard gauge, and are artificial, not true, inches.
115. Self-Registering Rain-Gauge.—The rain-gauge can be combined with clock-work and other mechanism so as to be self-recording of the amount of rain, the time, and duration of its fall. For the details of construction the reader is referred to the next chapter, where he will find the instrument described in connection with Osler’s anemometer, as the “pluviometer.” To observe and duly record the times of commencement and termination of rain is very desirable. Scarcely any observer can attempt to do this even approximately from personal observation. Hence the want of a cheap and simple self-recording rain-gauge is much felt, the present construction being too expensive for all but a few individuals.
In 1862, Mr. R. Strachan estimated the duration and amount of rain in London (Gray’s Inn Road) as follows:—
“During the year 1862, the rainfall amounted to 25·67 inches. Rain fell on 179 days, that is, on nearly every other day. The hours of rain were estimated at 904; therefore, if the rain had fallen continuously, it would have lasted nearly 38 days and nights.”[10]The value of similar estimates of the rainfall by numerous observers would be very great to meteorology.
116. The principle of measurementin all these gauges is the relation existing between the areas of the collecting and receiving surfaces; that is, between the area of the funnel into which the rain falls, and the area of the cylinder which receives it. In Howard’s and Glaisher’s gauges, this cylinder is virtually the measuring glass itself; in the others, above described, the measuring scales show the same depth of water as in the cylinder of the gauge.
The cylinder being of less diameter than the funnel, and receiving all the rain collected by the funnel, it follows that its contents will have an increased depth. Now equal cylindrical volumes, having different diameters, are to each other inlength inversely as the squares of the diameters. Hence, if the funnel be 9 inches and the cylinder 3 inches in diameter, a fall of 1 inch of rain will be represented in the gauge by 9 inches; for 3² : 9² : : 1 :x= 9. In this case, therefore, a length of nine inches of the measuring glass, tube, or scale, would represent an inch of rainfall, and be divided into tenths and hundredths of the artificial inch.
117. Position for Rain-Gauge, &c.—Rain gauges should be placed on the ground, in any position exposed to a free fall of rain, snow, or hail, where neither walls, buildings, nor trees shelter or cause eddies of wind. They should be supported by a frame, or other means, to prevent them being blown down by the wind, but so that they can be readily emptied.
During snow or frost, the gauge must be watched, and its contents melted by placing it in a warm room, either when the amount is to be measured, or the funnel is filled up with snow. A tin vessel of equal area to the funnel may at such times be useful as a substitute.
Rain gauges are constructed of metal, usually copper, which, besides being readily workable, is little affected by atmospheric influences. If made of iron or zinc, they should be well japanned; if of copper, this is not so essential. The capacity of a gauge should be sufficient to contain at least the probable maximum fall of rain in a day at the locality. Those required for rainy districts must be of large size.
118. Causes of Rain.—When the invisible vapour which is diffused in the atmosphere becomes sufficiently cooled, it appears visible as mist or cloud, and a further reduction of temperature causes its precipitation as rain, hail, or snow. The cooling of the higher regions of the atmosphere is doubtless the chief cause of this condensation; but the property which aqueous vapour possesses of radiating heat may also contribute to the result. Moreover, the law which regulates the amount of vapour which air at any particular temperature can sustain in a transparent state, determines that when two bodies of air at different temperatures, saturated with vapour, intermix, some moisture must be rendered visible; and hence, it is not only possible, but highly probable, that rain may result from the conflict of different winds. Let us imagine two cubic yards of air, both saturated with moisture, but having the respective temperatures of 50 and 70 degrees, to come into contact. There will be a tendency to equalize the temperature to a mean, which is 60°; and during this process, some of the vapour will be condensed.
It may be conceded, therefore, that when a warm and moist current of air encounters a body of cold air which may not be extremely dry, the mixture is unable to retain the whole of the vapour in an invisible state; so that the excess becomes visible as mist or fog, and, when the temperature has become sufficiently lowered, rain. The British Isles are more or less enveloped in fog, or mist, at the commencement of easterly winds, which, with a sudden change of wind, is exhibited even in summer; while the south-westerly winds, warm, and arriving from the ocean, deposit large quantities of rain by the cooling effect of the land, colder by reason of its latitude. When rain occurs with a northerly wind, it is probably due to the deposition from an upper south-westerly current, often apparently proved by the movements of the upper clouds.
119. Laws of Rain-fall.—Tropical countries have a dry and a wet season during the year:dry, when the sun is at the opposite side of the equator;wet, when the sun is overhead. With reference to the British Isles, the statistics collected by Mr. G. J. Symons indicate that: 1st. The stations of least rain are inland, or on the east or south-east coasts; the stations of greatest rain are on the western coasts. 2nd. The rain-fall is very large in the vicinity of mountain chains or groups, unless the station happens to be some miles to the north-eastward.
It may be well to illustrate these remarks by quoting[12]the average fall at a few places, grouping them as—
Mr. Green, the celebrated aeronaut, has asserted from his experience, “that whenever a fall of rain happens, and the sky is entirely overcast, there will invariably be found to exist another stratum of cloud at a certain elevation above the former;” and the recent scientific balloon ascents by Mr. Glaisher have tended to confirm this theory. Mr. Glaisher says, “It would seem to be an established fact, that whenever rain is falling from an overcast sky, there is a second stratum above.” “It would also seem that when the sky is overcast without rain, that there is no stratum of cloud above, but that the sun is shining on the upper surface. In every instance in which I have been up under these circumstances, I have found such to be the case, agreeing in this respect also with Mr. Green’s observations.”
The amount of rain collected in a gauge placed near the surface of the earth is larger than in any gauge placed above it; and the higher the gauge is placed, the less water is collected. Mr. Glaisher contends that his balloon experiments corroborate this law.
120. Utility of Statistics of Rain-fall.—The utility of knowing the rain-fall of any locality is sufficiently obvious, and little need be said upon the subject. The rain-gauge should be in the hands of every gardener and farmer. In the management of out-door plants and crops, as well as in the construction of cisterns and tanks for the supply of water, a rain gauge is a valuable assistant. By its use, the gardener will be guided in judging how far the supply of moisture to the earth is needed; and he will also see how beneficial is even a hasty shower to growing plants, when he considers that a fall of rain measuring the tenth of an inch in depth, corresponds to the deposit of about forty hogsheads per acre. The study of the rain-fall of a country is of considerable interest to agriculturists. The health and increase of domestic animals, the development of the productions of the land, as well as the daily labours of the farmer, are dependent upon the excess or deficiency of rain. “It must be a subject of great satisfaction and confidence to the husbandman to know at the beginning of a summer, by the certain evidence of meteorological results on record, that the season, in the ordinary course of things, may be expected to be a dry and warm one; or to find, in a certain period of it, that the average quantity of rain to be expected for the month has fallen. On the other hand, when there is reason, from the same source of information, to expect much rain, the man who has courage to begin his operations under an unfavourable sky, but with good ground to conclude, from the state of his instruments and his collateral knowledge, that a fair interval is approaching, may often be profiting by his observations; while his cautious neighbour who waited ‘for the weather to settle’ may find that he has let the opportunity go by. This superiority, however, is attainable by a very moderate share of application to the subject; and by the keeping of a plain diary of the barometer and rain-gauge, with the hygrometer and vane, under his daily notice.”[13]The statistics of rain-fall are not only valuable and interesting in a meteorological point of view, and for agricultural purposes, but are also highly important in connection with sanitary arrangements for towns, and engineering operations. This is especially evident to the hydraulic engineer. As rain is an important source of water-supply to rivers, canals, and reservoirs, it is evident that a knowledge of the probable fall for any season or month, at a given place, as furnished by averages of the observations of former years, will be the data upon which the engineer will base his plans for providing for floods or droughts; while the measurement of the actual quantity which has just fallen, as gathered from the indications of a series of gauges, will suggest to him the precautions to adopt either to economise or conduct away the in-pouring waters.
“When a canal is conducted across an undulating country, its course is necessarily governed by the accidents of the ground, and it alternately rises and falls. In this case, rising by a succession of levels, it necessarily arrives at a certain highest level, which is called by engineers thesummit level. From this it again descends by a corresponding series of levels. Now, it is evident that, supposing the locks to be all equal in magnitude, the ascent of a vessel will require the descent of as much water from the summit to the lowest level as would fill a single lock; for this quantity of water must be discharged from each lock of the series when the vessel passes through it.
“The same may be said of the process by which the vessel descends along the series of locks on the other side of the summit. It appears, therefore, that a supply of water must always be maintained on the summit level sufficient to fill a single lock twice for each vessel which crosses the summit.
“It happens, fortunately, that by the laws of natural evaporation, rain is precipitated in greater quantities on elevated summits than on the intermediate valleys, so that the moving power, in this case, accommodates itself to the exigencies of intercommunication.”—Dr. Lardner’s “Handbook of Natural Philosophy.”
121. New Form of Rain-Gauge.—Since the foregoing pages were in type, a modification of Howard’s rain-gauge has been arranged by Mr. Symons, which is compact in design, convenient in use, and low in price. It combines the advantages of most gauges; having solidity, and facility of measurement. The bottle is placed in a tin case, to the bottom of which are attached stout spikes, which, when forced into the earth, prevent its being upset either by wind or accident. The bottle being transparent, and slits made in the case, the fall of rain is seen at a glance, or with a race-glass, from a window. The funnel being attached to the cover of the case is thereby kept strictly horizontal, and the depth of rain can be accurately measured by lifting the bottle from its case and emptying it into a graduated glass jar.
The funnel of this gauge is a very deep cone, to prevent the rain drops outsplashing. When properly placed, the receiving surface will be twelve inches above the ground, which experience has shown to be the most advantageous height.
APPARATUS EMPLOYED FOR REGISTERING THE DIRECTION, PRESSURE, AND VELOCITY OF THE WIND.