CHAMBERS'S INFORMATION FOR THE PEOPLE. 



are used along with this hygrometer for calculating 

 the dew-point temperature, which is ascertained, 

 without calculation, by Daniell's instrument.* 



The force of evaporation, or the quantity of 

 water which is converted into invisible vapour in 

 a given time, depends upon the dryness of the 

 air, and upon its stillness or motion. The effect 

 of a current of air is to carry off the vapour from 

 the surface as it rises, and leave the space free 

 for an additional supply. The more rapid the 

 wind, the faster every wet surface dries up. 



Clouds. We must now consider more par- 

 ticularly the peculiar forms of visible vapour. 

 Mist, fog, and cloud are names for nearly the 

 same thing. When there is a general haze of 

 precipitated vapour covering the whole sky, and 

 coming down to the surface of the earth, it is 

 termed a fog. When a white smoke is seen 

 to rise from the courses of rivers and wet land, 

 it is called a mist; but mist and fog are often 

 indiscriminately applied to the same appearance 

 namely, to a vapoury haze lying upon the 

 ground. Clouds are the masses of haze or fog 

 which are seen floating in the higher regions, 

 and which do not descend to the ground. 



The steam of a kettle when it becomes visible, 

 the white cloud of a steam-chimney, our breath in 

 a moist or cold day, are all precipitated moisture 

 or mist. Sometimes this mist is rapidly reab- 

 sorbed as it spreads out, shewing that there is a 

 certain dryness or vacancy in the air sufficient to 

 take it in among the invisible steam, when it is 

 distributed over a somewhat larger surface. 



If the air is exposed to a cooling, so as to bring 

 it beneath the temperature of the dew-point, vapour 

 must be thrown out everywhere, and a fog or mist 

 will be formed. In this manner are formed the 

 dense fogs of the polar seas, the fogs found 

 along the courses of rivers, upon the sides 

 of mountains, and over shoals and headlands. 

 The celebrated fogs of London originate in the 

 same way ; but their black and thick appearance 

 is owing to the quantity of smoke which is sus- 

 pended in them. 



The formation of clouds, as well as their 

 frequent disappearance, can, in most cases, be 

 readily explained by the general principles regard- 

 ing the atmosphere and its vapour already laid 

 down. Thus, we often see a clear morning gradu- 

 ally become overcast with clouds. This arises 

 from the sun's rays heating the earth, and causing 

 currents of warm, moist air to ascend into regions 

 where it is cooled by expansion below the dew- 

 point, and part of its vapour is condensed into 

 clouds. As the day declines, the process is 

 reversed ; the cloudy particles descend and are 

 dissolved, and a cloudless evening ensues. 



It is not uncommon to see the top of a hill 

 wrapped in a stationary mist or cloud, even while 



* Besides the above, there are various instruments called 

 hygrometers, depending on the principle of the shrinking and 

 expanding of bodies in relation to the degree of humidity with 

 which they are affected. Fibrous vegetable substances, such as 

 ropes, contract by imbibing moisture ; while, on the contrary, 

 hairs and catgut (strings of violins) contract by drought. Hair 

 has been found to be the most delicate in hygrometrical motions. 

 Saussure accomplished the construction of a hygrometer from 

 a single long hair, previously cleaned in a soda lye. Various 

 philosophical toys, as ornaments for mantel-pieces, have also 

 been constructed to indicate the dryness and moistness of the j England the mares-tail Cloud. The CUmulUS IS 



a pretty rapid breeze is blowing over it. The 

 explanation is simple. The current of air, in 

 crossing the hill, rises into a region where the 

 temperature is below its dew-point, and part of its 

 vapour is there condensed into mist ; but, as it 

 descends the other side, it becomes warmer, and 

 can again dissolve the watery particles. The sub- 

 stance of the cloud is thus really in motion over 

 the summit ; but it is constantly being shorn off 

 on the lee-side, and as constantly receiving addi- 

 tions on the other side, so that it appears motion- 

 less as a whole. 



A question often asked regarding clouds is thus 

 answered by Mr Buchan in his valuable Handy 

 Book of Meteorology : 'A very natural inquiry is : 

 How are clouds suspended in the air ? The 

 example of a cloud appearing to rest on the top 

 of a hill though a strong wind be blowing at the 

 time, suggests an explanation. The cloud itself 

 may appear stationary or suspended, but the par- 

 ticles of which it is composed are undergoing con- 

 stant renewal or change. The particles are upheld 

 by the force of the ascending current in which they 

 are formed ; but when that current ceases to rise, or 

 when they become separated from it, they begin to 

 fall through the air by their own weight, till they 

 melt away and are dissolved in the higher tempera- 

 ture into which they fall. Hence, as Espy has reas- 

 oned, every cloud is either a forming cloud or a 

 dissolving cloud. While it is connected with an 

 ascending current, it increases in size, is dense at 

 the top, and well defined in its outlines ; but when 

 the ascending current ceases, the cloud diminishes 

 in size and density.' 



The classification of clouds proposed by Luke 

 Howard in 1803, is still universally followed. The 

 three principal forms are the cirrus, or feather- 

 cloud, the cumulus, or heaped-cloud, and the 

 stratus, or stretched-cloud. The cirrus is corn- 



Cirrus. 



posed of thin threads or filaments, aggregated 

 into woolly or feathery forms, and sometimes 

 making a delicate, slender net-work. It is the first 

 indication of serene and settled weather. The 

 duration of the cirrus is uncertain from a few 

 minutes to several hours. It lasts longer, if it 

 appears alone, and at a great height. From its 

 usually curling appearance, the cirrus is called in 



atmosphere all on the similar principle of contraction and 

 expanaon of a hau-, p.ece of catgut, or part of the beard of the 



36 



the kind of cloud resembling mountains piled upon 

 mountams> an( j gener ally ends above in rounded 



