The theory of the trade-winds was first propounded by George Hadley in the Philo .sophical Transactions for 1735; and it may be mentioned as a remarkable fact that for about half a century it remained quite unnoticed, when it was independently arrived-at by Dalton, and published in his essays.
The publication of Dalton's Meteorological Essays, in 1793, marks an epoch in mete orology. It is the first instance of the principles of philosophy being brought to bear on the explanation of the intricate phenomena of the atmosphere. The idea that vapor is an independent elastic fluid, and that all elastic fluids, whether alone or mixed, exist independently; the great principles of motion of the atmosphere; the theory of winds, their effect on the barometer, and their relation to the temperature and rain; observations on the height of clouds, on thunder, and on meteors; and the relations of magnetism and the aurora borealis,' are some of the important questions discussed in these remarkable essays, with an acuteness, a fullness, and a breadth of view that leave little to be desired: One of the most interesting and truthful subjects of inquiry that engaged the atten lion of meteorologists was deto. The observations on this subject were first collected and reduced to a perfect theory by Dr. Wells. See DEW.
In 1823 Daniell published his Meteorological Essays and Observations, which, while adding largely to our knowledge in almost every department of the subject, are chiefly valuable as bearing on the hygrometry of the atmosphere. Though the_pmctical advan tages which he anticipated would flow from it have not been realized, yet this diffi cult and still obscure department of meteorology stands indebted to him more than to any- other philosopher. The law of the diffusion of vapor through the air, its influence on the barometric pressure, and its' relations to the other constituents of the atmosphere are among the least satisfactorily determined questions in meteorology. Since this ele ment is so important as an indicator of storms and other changes of the weather, and .since so much remains still to be achieved, it is to be hoped that it will soon be more thoroughly investigated. A most important addition has lately been made to our knowl edge of the vapor of the atmosphere by professor Tyndall, in his experiments on radiant heat, especially as regards the gases. The vapor of water is there shown to exert extraor dinary energy as a radiant and absorbent of heat; and hence the vapor dissolved in the air acts the part of a covering or protection to the earth. As it is, to some extent,
impervious to solar and terrestrial radiation, it follows that if the air were quite drained of its moisture, the extremes of heat and cold would be so intense and insufferable that all life would instantly perish, there being no screen shielding the earth from the scorching glare of the sun by day and from the equally scorching and blighting effects of its own radiation by night. It is to be expected that this great discovery will soon throw light on many questions of meteorolog,y.
Electrical observations have been, of all meteorological observations, perhaps the 'least productive, partly owing to their scantiness, from the expense and trouble attend ing them, and partly, no doubt, to the free and bad use made of the name of electricity "by crude theorists in explaining phenomena of which it would have been wiser to have confessed their ignorance. But the brilliant discoveries which have recently been made on the mutual relations of heat, motiou, electricity, magnetism, and the other forces of matter, lead us to indulge the hope that the application of these results to meteorology will be attended with discoveries equally brilliant and-important. , Humboldt's treatise on Isothermal Lines (181'7) constitutes a notable epoch in experi mental meteorology. Dove has since continued the investigation, and in his splendid work, Om the Distribution of Heat on the Suifac,e of the Globe, has given charts of the world, showina the temperature for each month and for the year, and also charts of abnormal tempeatures. It -is scarcely possible to overestimate the value of this work, for though, to a considerable extent, the lines are hypothetical, there can be no doubt that a close approximation to the march of mean temperature and its distribution over the earth tluough the year has been arrived at. The idea has been carried out with greater fullness of detail by the United States government in the beautiful and elaborate series of charts of tem perature and rainfall given iu the Army _Meteorological Register for 1855. In these charts the temperature and rainfall in the different seasons for every part of the United States, deduced from accurate observations, may be seen at a glance. Buchan has published isothermals for the British isles, Mohn for Norway, and Blandford for Hindustan; and isothermals for the sea have been published by the admiralty.