Page:Physical Geography of the Sea and its Meteorology.djvu/181

Rh until more numerous and accurate observations shall better satisfy the theory than those half a million and more now do, be regarded as the sole cause of the easterly direction of the trade-winds. It suggests either that other agents are concerned in giving the trade-winds their easting, or that the effect of the upper and counter current, when drawn down and turned back (§ 232), is such as to counteract their unequal turning in obedience to the varying forces of diurnal rotation. No apology is needed for applying the tests of actual observation to this part of the Halleyan theory, notwithstanding the general concurrence of opinion as to its sufficiency. With equal favour that feature of it also was received which ascribes the rising up in the belt of equatorial calms to the direct influence of the solar ray. But the advancement which has been made in our knowledge of physical laws since Halley expounded his trade-wind theory suggested a review of that feature, and it was found that, though the direct heat of the sun is one of the agents which assists the air to rise there, it is not the sole agent; the latent heat which is set free by condensing vapour for the equatorial cloud-ring and its rains is now also (§ 252) recognized as an agent of no feeble power in this calm belt.

345. Faraday's discovery of magnetism in the air.—Where shall those who are disposed to search, look for this other agent that is supposed to be concerned with the trade-winds in their easting? I cannot say where it is to be found, but considering the recent discoveries in terrestrial magnetism—considering the close relations between many of its phenomena and those both of heat and electricity—the question may be asked whether some power capable of guiding "the wind in his circuits" may not lurk there? Oxygen comprises more than one-fifth part (two-ninths) of the atmosphere, and Faraday has discovered that oxygen is para-magnetic. If a bar of iron be suspended between the poles of a magnet, it will arrange itself axially, and point towards them; but if, instead of iron, a bar of bismuth be used, it will arrange itself equatorially, and point in a direction perpendicular to that in which the iron pointed. To distinguish these two kinds of forces. Dr. Faraday has said iron is para-magnetic, bismuth dia-magnetic. Oxygen and iron belong to the same class, and all substances in nature belong to one or the other of the two classes of which iron and bismuth are the types.