Page:Popular Science Monthly Volume 27.djvu/307

Rh It is obvious that this action of extensive reflecting surfaces of snow must exert a powerful influence upon the maximum temperature of places favorably situated for receiving the reflected rays; and, moreover, where the proportion of heat reflected varies (as it has been proved to do in the case of water, and as it doubtless also does in the case of snow) inversely as the angle formed by the incident rays and the reflecting surface, this action must materially contribute, especially in winter, to the maintenance of an approximately uniform sun-temperature throughout the day. At Davos, and similar elevated stations, however, the comparative freedom of the air from suspended particles must, to a great extent, contribute to such a result; for, as pure and dry air is transcalent and reflects heat but very slightly, the horizontal sunbeams, passing through such air, would be nearly as powerful as vertical rays.

The peculiar winter climate of Davos depends, therefore, upon the following: conditions:

—This single condition favors a genial and wholesome winter climate in several ways. In the first place, by reducing the weight of cold air in contact with the body, whereby, even with a much lower thermometer, the air, if still, feels warmer at an elevated station than in the lower and denser regions of the atmosphere, in consequence of the slower abstraction of heat from the body. In the second place, the air at great altitudes is more permeable to the heating rays of the sun, owing both to its dryness and to its freedom from dust and suspended particles generally. In illustration of this I have made many experiments, chiefly in England and Switzerland, and an abstract of the results obtained is given in the following table:

It is thus evident that, although the air-temperature continually decreases as we ascend, the sun-temperature as regularly augments. The horizontal line in the table divides the observations into two groups, in each of which the sun's altitude was approximately the same. In these, and similar observations described further on, the air-temperature was found by placing an ordinary mercurial thermometer upon a