Page:Popular Science Monthly Volume 21.djvu/770

752 mountains they must be exposed, even in the sunshine, and certainly in the night, when the solar heat is not poured upon them, to temperatures colder than those of the frigid zones.

As they occur in all climates, over the poles as well as at the equator, 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 (condensable) moisture in the cloud than in the surrounding air. And aëronauts 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 special 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, although 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 well-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 departments of the various nations where the observers are at times in the midst of clouds, opportunities may occur for observation and examination 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.