-1859] WRITINGS OF JOSEPH HENRY. 51 



which have been reduced by it, that they afford very satis- 

 factory corresponding results. We propose to give a fuller 

 discussion of this part of the subject in another report. 



Motions of the Atmo^here. — The repulsion of the atoms of 

 the air is not only increased by a diminution of distance 

 from being pressed closer together, but also by an addition 

 of heat. From the latest and most reliable experiments on 

 this point it is found that the pressure being the same, air 

 expands ^i-r part of its bulk at the freezing point for each 

 degree of Fahenheit's scale. Heated air therefore becomes 

 specifically lighter, and tends constantly to ascend, being 

 pressed upwards by the heavier circumambient fluid. The 

 effect thus produced upon the air by the impulses from the 

 sun is the great motive power which gives rise to all the 

 currents of the atmosphere, from the gentle zephyr which 

 slightly ripples the surface of the tranquil lake to the raging 

 hurricane which overwhelms whole fleets, or destroys in a 

 moment the hopes of the husbandman for an entire season. 

 This fact is so well established by science that it is unnecessary 

 to seek for any other primum mobile for the great system of 

 constant agitation to which the aerial ocean is subjected. 



Allowing the temperature of the equator, on an average, 

 to be 82° F., that of the pole zero, and of the top of the air, or 

 in other words, of celestial space, to be — 60°, and estimating 

 the height of the atmosphere at 50 miles, it will follow from 

 the law of expansion by heat, that the excess of elevation of 

 the air at the equator will be upwards of four miles above 

 that of the pole. Although this is not intended to present 

 the exact amount of the aerostatic pressure, yet it will serve 

 to show the great motive power constantly maintained by 

 the influence of the solar radiation. In order to simplify 

 the conception of the motions which result from this dis- 

 turbing power, let us in the first place, suppose the earth to 

 be at rest, and its whole surface of a uniform character, con- 

 sisting, for example, of water. It is obvious from well estab- 

 lished hydrostatic principles, that the air expanded as we 

 have stated at the equator, would flow over at the top and 

 descend, as it were, along an inclined plane towards the 



