752 THE POPULAR SCIENCE MONTHLY. 



mountains they must be exposed, even in the sunshine, and certainly 

 in the night, when the solar heat is not poured upon them, to temper- 

 atures colder than those of the frigid zones. 



As they occur in all climates, over the poles as well as at the equa- 

 tor, and even in the warm latitudes at elevations which are above the 

 regions of unmelted snows, it must be assumed that a low temperature 

 alone does not cause them to give up their moisture in the form of rain 

 or snow. 



Glaisher, in entering a cloud eleven hundred feet thick, found the 

 dew-point remained unchanged, showing that there was no more (con- 

 densable) moisture in the cloud than in the surrounding air. And 

 aeronauts obtain no dew by Regnault's hygrometer at an elevation of 

 five miles, but clouds float above that height. 



The moisture in the air must not be confounded with the water of 

 the clouds. This moisture is precipitated by a low temperature, as is 

 seen in the condensation of water on the outside of a glass of ice-water 

 on a warm day, and the coating of the inside of window-panes with 

 ice on a very cold one ; and the formation of dews in summer and of 

 frosts in winter. 



The precipitation of the moisture from clouds must be caused by 

 some peculiar condition of the clouds themselves. After a rain there 

 often are as many clouds remaining or passing away as there were at 

 the commencement of or at any time during the rain. In this there is 

 evidence that the action going on in a part of a single cloud, or in spe- 

 cial clouds, does not extend over the whole mass, nor to other clouds 

 near by. 



In countries where it seldom or never rains, or where the rains are 

 periodic, clouds are as common as in places where it rains often. From 

 this it appears that the causes which produce precipitation are entirely 

 suspended, or rather do not exist in certain localities and seasons, al- 

 though clouds abound there at the time. 



Another fact worthy of consideration is that snow and rain fall 

 slowly, little by little, and not in one sudden down-pour, as would be 

 the case if the whole mass of cloud or clouds were brought at the same 

 instant under the action which produces rain or snow. 



It must not be forgotten that clouds move in w T ell-defined masses, 

 sometimes retaining their shape for a long time, and do not disappear 

 in the air, as they would do if they were ordinary water-vapor. 



As it is difficult to reach the clouds, little has been learned of their 

 composition. But at the elevated stations of the meteorological de- 

 partments of the various nations where the observers are at times in 

 the midst of clouds, opportunities may occur for observation and exam- 

 ination of them, which will reveal the laws by which they are governed. 

 It is not too much to expect that the acute and practical minds of the 

 present age will, ere long, add much to our present scant knowledge 

 of these mysterious meteors. 



