THE SMITHSONIAN INSTITUTION. 197 



by the diminished pressure, two cubic feet will onl}' weigh about as 

 much as one did at the surface of the earth. Two cubic feet, there- 

 fore, would contain 1.6 grains of water. Now, it will be evident, if this 

 upper air was to descend to the surface of the earth, it would have its 

 temperature raised by coming under increased pressure ; but its dew 

 point, as will be seen by inspecting a table, would be no higher than 

 23°, instead of 61°; the radiation during one night would, therefore, 

 cool down the air at the surface of the earth to this temperature. The 

 dry uj^per current coming over the Rocky Mountains oiten descends to 

 the earth in winter, and is the cause of the severe weather at that 

 season in the country east of these mountains. The great westerly 

 current of air from the Pacific in passing over the Rocky Mountains, 

 and in ascending their westerly slopes, becomes colder by expansion, 

 and, consequently, deposits its moisture. It is true the heat liberated 

 by the condensation of the vapor will tend to elevate the temperature of 

 the air on the top and at the foot of the mountain above its nominal 

 point; but this effect is counteracted, as we have said, by the increased 

 freedom the dry air gives to the radiation of heat from the surface of 

 the earth and the lower stratum of air, particularly when it is spread 

 over a wide extent of radiating surface. The descent, therefore, of the 

 dry and upper current of westerly winds to the surface of the earth is. 

 I think, the principal cause of the sudden and extreme changes of 

 weather in this clime. President D wight states that in the month of 

 July, 1804, considerable snow fell at Salem, in New England, and that 

 a severe frost was experienced in different parts of the same country. 

 In all parts of the world where moist winds, or those from the ocean, 

 blow over high ranges of mountains, they deposit their moisture in the 

 form of rain and snow on the windward slopes. This is the case on 

 the coast range of Oregon and California, and also on the slopes op- 

 posite the prevailingmoist winds in South America. 



In England not more than 23 inches of water falls annually in some of 

 the level eastern counties during the year, while nearly 200 inches 

 occasionally fall on the western side of the mountains of Cumberland. 

 The west wisds of Norway are remarkably mild and rainy for such a 

 high latitude, while the same winds in Sweden are comparatively 

 cold ; the air, in losing its moisture on the high chain of mountains, 

 loses its power of retaining the large amount of latent heat extricated 

 by the condensation of vapor. 



The production of cold through expansion is the cause of clouds 

 resting on the tops of mountains when they should apparently be borne 

 away on the breeze. The precipitation of vapor as the air rises on the 

 one side, and the evaporation of it as the air descends on the other, is 

 the true cause of the phenomenon. The formation of clouds in the 

 sky and precipitation of moisture are chiefly produced, as Mr. Espy 

 has demonstrated, by the asceni of comparatively moist and warm 

 air from below. The expansion produces cold, and of course the pre- 

 cipitation of moisture into clouds and rain. The cumulus clouds of 

 summer are thus formed by ascending currents of moist air. The con- 

 densation of the vapor extricates a large amount of latent heat, which 

 expands the air within the cloud, and thus produces an increased 

 buoyancy. Before thunder-storms the air is usually moist and oppres- 

 sive, the perspiration from the skin is checked by the moisture, and 



