Jan. 3, 1889] 



NATURE 



225 



■ aly carries off the excess, no ill conseqtience may ensue. 

 Again, if a kettle be filled with ice-cold water and put on 

 I gas stove, suppose it takes ten minutes to bring it to 

 i )oil In that ten minutes the water has absorbed as much 

 heat as raises it from 32' to 212", an increase of 180 . 

 Now, if it be left boiling, the gas-flame being kept up at 

 the same intensity, we may assume that in every succeed- 

 ing ten minutes the same quantity of heat is being 

 absorbed by the water. But it gets no hotter : it gradually 

 boils away. And it takes nearly an hour, or more than five 

 times as long as it took to heat it, before the whole of the 

 water has boiled away, since all this heat has been used 

 up in converting it into steam. It was by an experiment 

 of this kind that Dr. Black, in the last century, discovered 

 the fact of latent heat, and determined its amount ; and it 

 was the knowledge of this fact that led James Watt to his 

 first great improvement in the steam-engine. 



One more example I may give, which those who 

 have been in India will be able to appreciate, and which 

 those who intend to go there may some day find useful to 



ow. Nothing is more grateful in hot dry weather than 



drink of cold water. Now, ice is not always to be had, 



t when a hot wind is blowing, nothing is easier than to 

 et cold water, if you have a pot or bottle of unglazed 



rthenware, such as are to be had in every bazaar, or, 

 ■what is better, a leather water-bottle, called a chhagal, or a 

 water-skin. All these allow the water to soak through 

 and keep the outside wet ; and if any one of them be 

 filled with water and hung up in a hot wind, in the course 

 of half an hour or an hour, the evaporation from the out- 

 side will have taken away so much heat that the contents 

 •may be cooled 20' or 30, notwithstanding that the ther- 

 mometer may stand at no" or 115° in the shade. Soda- 

 water may be cooled in the same way if wrapped in straw 

 and kept well wetted while exposed to the wind. But it 

 is of little use to do as I have seen natives do sometimes, 

 viz. put the bottles into a tub of water in a closed room. 

 It is the evaporation that carries off the heat, otherwise 

 the water is no cooler than the air around. 



Now to return to our subject. The atmosphere always 

 contains some vapour which the winds have taken up 

 from the ocean, lakes, rivers, and even from the land, for 

 there are but few regions so dry and devoid of vegetation 

 that there is no moisture to evaporate. The quantity of 

 water thus evaporated from large water surfaces is a 

 •question of some importance to engineers, who have to 

 take account of the loss from reservoirs and irrigation 

 tanks, and a good deal of attention has been given to 

 measure the amount lost by evaporation. In England it 

 has been found to vary in different years from 17 to 27 

 inches in the year, or say from li to 2| inches per month 

 on an average. Now, since in the east of England the 

 rainfall is only about 24 inches in the year, it follows that 

 in that part of the Kingdom the loss by evaporation from 

 a water surface is not very much less than the rain falling 

 •directly on the surface. 



In dry countries the evaporation may exceed the local 

 rainfall. In the tropics it has been found to average from 

 3.J to 6 inches per month in the dry season. In the case 

 of a large tank at Nagpur, constructed to supply the city 

 with water, it was found that the loss by evaporation, in 

 the hottest and driest weather, was two and a half times 

 as great as the quantity supplied for consumption. 



These statistics will give some idea of the enormous 

 evaporation that goes on from the water surfaces of the 

 globe, and to this must be added all that takes place from 

 the land. In the case of light showers, neaily the whole 

 of the rain is re-evaporated ; and probably, on an average, 

 half of the total rainfall on the land is thus lost sooner or 

 later, leaving not more than half for the supply of springs 

 and rivers. 



The quantity of vapour in the air is very variable. 



To us, in England, the west and south-west winds are the 



', dampest, coming direct from the Atlantic, and north-east 



winds are the driest. The cause of their extreme dryn es 



I shall endeavour to explain presently. It is no doub 

 partly due to the fact that they reach us from the land 

 surface of Europe, but partly also to another cause to 

 which I shall have to advert later on. 



The quantity of vapour in the air is usually ascertained 

 by the hygrometer, the ordinary form of which is a pair of 

 thermometers, one having the bulb wet, the other dry, and 

 observing the depression of the wet bulb. The principle 

 of this I have already explained. But the same thing 

 may be ascertained more directly by passing a measured 

 quantity of air through a light apparatus containing 

 sulphuric acid, or some other substance that absorbs 

 water vapour greedily, and weighing the whole before and 

 afterwards. The increase of the second weighment gives 

 the weight of water absorbed. By such means it has 

 been ascertained that air at 60" can contain as much as 

 5 1 grains of vapour in each cubic foot, and that air at 80° 

 can contain rather less than 1 1 grains in the same space. 

 The quantity that air can hold increases therefore very 

 rapidly with the temperature. But it is seldom that it 

 contains this maximum amount, especially at the higher 

 temperatures. 



In order to condense any part of this vapour we must 

 take away its latent heat. It is not sufficient merely to 

 cool it till it reaches the temperature of condensation, but 

 we have further to abstract 5^ times as much heat as would 

 raise the condensed water from the freezing to the boiling 

 point. Before, however, proceeding to consider how this 

 cooling ii effected, the question arises, What is the con- 

 densing point ? For, obviously, since water can evaporate 

 at all temperatures, so we should expect that it may 

 condense at all temperatures. On what, then, does the 

 condensing point depend ? 



I mentioned just now that air at the temperature of 60° 

 can contain as much as 5^ grains of vapour, and at 80° 

 rather less than 1 1 grains in each cubic foot. Obviously, 

 then, if air at 80°, contaming this maximum quantity, be 

 cooled to 60", it must get rid of more than 5 grains, or nearly 

 half its vapour, and this excess must be condensed. I speak 

 of air containing these quantities, but in point of fact it 

 makes no appreciable difference whether air be present or 

 not. An exhausted glass vessel of one cubic foot capacity 

 can hold 5I grains of vapour at 60' and no more, and nearly 



I I grains at 8o~ and no more ; and if, when thus charged at 

 80', its contents be cooled to 60"", more than 5 grains will 

 he condensed. If, however, it contain only 5I grains at 

 80^, none will condense until the temperature falls to 60% 

 but any further cooling produces some condensation. 

 Thus, then, the condensing point depends on the quantity 

 of vapour present in the air, and is the temperature at 

 which this quantity is the maximum possible for that 

 temperature. 



This preliminary point being explained, we may now 

 proceed to inquire what means Nature employs to con- 

 dense the vapour in the air, producing at one time dew 

 and hoar-frost, at another time fog and cloud, and at 

 another rain, hail, and snow. 



Let us take the case of dew and hoar-frost first, as they 

 are comparatively simple. And in connection therewith 

 I may relate a little incident that took place at Calcutta 

 some years ago. A gentleman, who had not much ac- 

 quaintance with physical science, was sitting one evening 

 with a glass of iced brandy and water before him. It was 

 in the rainy season, when the air, though warm, is very 

 damp, and he had a large lump of ice in his tumbler. On 

 taking it up, he noticed to his surprise that the glass was 

 wet on the outside, and was standing in quite a little pool 

 of water on the table. At first he thought his tumbler 

 was cracked, but putting his finger to his tongue he found 

 the fluid tasteless. " Very odd ! " he remarked ; " the water 

 comes through the glass but the brandy doesn't." 



Now, however with our present knowledge we may be 

 inclined to smile at the simplicity of this remark, it so 



