104 



THE POPULAR EDUCATOR. 



METEOROLOGY. II. 



AQUEOUS METEOBS MOISTUR3 OF THE AIR HYGROMETERS 

 MIST DEW CLOUDS. 



IN our present lesson we propose to turn our attention to the 

 aqueous phenomena of the atmosphere. The air, as has been 

 explained, consists mainly of two gases, oxygen and hydrogen ; 

 there is, however, always present in it a greater or less amount 

 of watery vapour, and this gives rise to the various phenomena 

 of which we are about to treat. 



If we expose a vessel containing water to the air, sheltering 

 it sufficiently to prevent rain falling into it, we shall find that 

 in the course of a few days it will be empty, the water having 

 completely evaporated. By weighing the vessel before, and 

 again after a few hours' exposure, we shall be able to obtain 

 proof of the same fact in a much shorter time, and shall also be 

 [ able to estimate approximately the rate at which evaporation 

 has been going on. Now try a similar experiment 

 with snow, and it will be found that, even when the 

 temperature is below the freezing point, it slowly 

 evaporates. This explains why it is that snow slowly 

 disappears, even during the continuance of a frost. 



Experiments of this kind the student should try 

 for himself, as he will thus acquire more practical 

 information than can well be acquired from books, 

 and, at the same time, will become proficient in 

 interrogating Nature, and making trustworthy obser- 

 vations. The latter point is of the utmost importance, 

 since many observations, which 

 would otherwise be very valuable, 

 are rendered entirely useless by 

 inaccuracies that have crept into 

 them through want of care. 



It is a difficult matter to a"s- 

 certain exactly the amount of 

 evaporation which takes place 

 from the surface of the earth. It 

 varies, of course, with distance 

 from the equator, and various 

 other causes, but is estimated in 

 Great Britain to be equal to an 

 average depth of thirty-two inches 

 of water. Of this amount, less 

 than three inches is raised in the 

 months of November, December, 

 January, and February ; while in 

 June, when the maximum is 

 attained, it amounts to nearly 

 four inches. In tropical regions, 

 the quantity is estimated to be 

 from 90 to 100 inches. 



The amount of watery vapour 

 that can thus be held by the 



air is limited. It varies, however, with the temperature, and 

 to this mainly the various phenomena of clouds and rain are to 

 be attributed. It may be well to remember that air at 32 can 

 contain the 160th part of its orm weight of water in the state of 

 vapour, and that this amount is doubled by every 27 rise in 

 temperature ; so that at 59 air can contain the 80th part, and 

 at 86 the 40th part. It is, however, very rarely that the air is 

 fully saturated, that is, contains as much vapour as it can 

 take up. 



Various instruments have been devised for the purpose of 

 ascertaining the humidity of the air ; these are known as hygro- 

 meters. The most common of these is Saussure'a Hair Hygro- 

 meter, which is represented in Fig. 2. 



This instrument consists of a long hair, c, carefully cleaned 

 from fat by being treated with ether. One end of it is fixed in 

 a clamp, d, capable of being moved by a small screw, 6, so as to 

 adjust the index. The other end is fastened to a pulley, o, which 

 carries the hand. A small weight, p, keeps the hair tight. As 

 soon as the hair attracts moisture from the air it elongatos ; the 

 hand is then moved down by the weight, and shows on the 

 scale the degree of moisture. As the air becomes drier, the hair 

 contracts again, and raises the index. 



Another hygrometer, acting upon a different principle, is re- 

 presented in Fig. 3. This instrument, which is known as 

 Daniell's. consists of a bent glass tube, with a bulb blown at 



each end. The tube is partly filled with ether, which has been 

 boiled in it so as to expel the air, and inside A is a deli- 

 cate thermometer, the bulb of which dips into the ether. B is 

 wrapped round with some muslin, and ether is dropped on this 

 when it is required to make an observation. This ether rapidly 

 evaporates, and thus lowers the temperature of B, and some of 

 the ether from A at once begins to condense in it. The tem- 

 perature of A accordingly falls, and this bulb is watched till 

 the moisture of the air begins to condense on it. As soon as 

 this occurs, the thermometer in it is read, while that in the 

 stand gives the temperature of the air at the same time. When 

 the air is nearly saturated, the difference between the two will ba 

 but slight, but the drier the air the greater the difference. 



The reading of the thermometer in A gives us, in fact, the 

 dew-point ; that is, the temperature at which the air becomes 

 saturated, and begins to part with some of its moisture. The 

 humidity can be easily calculated from this. 



Another form of hygrometer consists of two ther- 

 mometers placed side by side (Fig. 4) ; the bulb of 

 one is covered with muslin, which is kept wet by 

 capillary attraction, a small vessel of water being- 

 placed near it, into which the muslin or some 

 threads of cotton connected with it dip. The differ- 

 ence between the readings of the two will give the^ 

 moisture of the air. 



True watery vapour is quite invisible ; but when 

 the air is fully saturated, and it begins to condense, 

 it becomes visible, and takes the form of fogs, mists, 

 or clouds. The usual cause of 

 fogs is the moist soil being at a 

 higher temperature than the air 

 above it. Vapours then continue 

 to arise from the ground, but are 

 speedily condensed by the cooler 

 air, and hang over the soil. 



Often during a clear night the 

 surface of the ground becomes 

 much cooled by radiation ; the air 

 lying on it therefore falls in tem- 

 perature, and becomes cooler than 

 that above it. If the ground be 

 level, no fog will be caused ; but 

 if it be sloping, the cold air will 

 sink by its weight to tho lower 

 level, and in doing so will con- 

 dense the moisture of the warmer 

 air with which it comes in 

 contact. This will accordingly 

 assume tho form of mist. 



The presence of a lake or river 

 in the valley causes the air to 

 be more saturated, and thus in- 

 creases the amount of mist, which, 



in this case, will often seem to lie along the surface of the water. 

 Elvers which flow from colder into warmer latitudes as, for 

 example, the Mississippi are colder than the air above them, 

 and hence condense its moisture into mist. A very similar 

 effect to this may be observed by placing a largo lump of ioe in 

 a warm room. Vapour will appear to arise from it, the real 

 fact being that the cold given off causes the vapour in tho air 

 around to assume a visible form. 



If a stream be warmer than the air around, a simile,:: effect 

 will be produced, the quantity of vapour given off by it being 

 greater than can be dissolved in the air above it. The fogs 

 which prevail along the course of the Gulf Stream illustrate this 

 fact clearly. 



The densest fogs occur during tho winter months in large 

 towns built near rivers. It is believed that the electrical con- 

 dition of the air influences them to a considerable extent. The 

 main cause, however, is that the abundant moisture of the air is 

 condensed by the cold winds. The amount of this moisturo is, 

 of course, much augmented by the breath of the many inha- 

 bitants ; and in manufacturing places the smoke becomes 

 entangled with the fog, and adds to its opacity. 



The tops of mountains are very frequently observed to be 

 covered with clouds, which they are supposed to attract. The 

 real reason of this is that, owing to their elevation, they are 

 much cooler than the air which passes by them, and therefore 



