Page:Popular Science Monthly Volume 43.djvu/410

396 another. The velocity of rotation of the atmosphere in tropical latitudes must therefore lag behind the rotation of the earth, and it must outstrip it in higher latitudes, mathematical calculation proving that the thirty-fifth parallel is, in both hemispheres, the line of division between the two. The general system of air circulation deduced from these principles is very similar in its results to the system of Ferrel; but the interest and importance of Siemens's views lie elsewhere. His memoirs were an appeal and an attempt to apply the principles of thermodynamics to the aërial currents, and they have opened the way for a series of important researches, which, however, are not yet sufficiently advanced to be discussed in these pages.

And, finally, a third new point of view has been introduced into the same discussions by Helmholtz. Sitting one day by the seaside, and observing how wind blows on the surface of the sea by sudden gushes, how it originates waves, and how they grow when wind blows with an increasing force, Helmholtz came to consider what would happen with two air currents blowing one above the other in different directions. A system of air waves, he concluded, must arise in this case, in the same way as they are formed on the sea. The upper current, if it is inclined toward the earth's surface (as is often the case), must originate in the lower current immense aërial waves rolling at a great speed. We do not generally see them, but when the lower current is so much saturated with moisture that clouds are formed in it, we do see a system of wavelike parallel clouds, which often extend over wide parts of the sky. To calculate the sizes. of the waves in different cases is extremely difficult, if not impossible; but by taking some simpler cases Helmholtz and Oberbeck showed that when the waves on the sea attain lengths of from sixteen to thirty-three feet, the air waves must attain lengths of from ten to twenty miles, and a proportional depth. Such waves would make the wind blow on the earth's surface in rhythmical gushes, which we all know, and they also would more thoroughly mix together the superposed strata, dissipating the energy stored in strong currents. These views are so correct that they undoubtedly will throw some new light, as they already begin to do, upon the theory of cyclones.

At the same time, Bezold is now endeavoring to reconstruct meteorology from the point of view of thermodynamics; and the well-known Austrian meteorologist, J. Hann, whose work is