664 
PROFESSOR JAMES THOMSON ON THE 
variety of vortex motion, constituting what we may call a vortex of free mobility in 
a frictionless fluid, and to offer a dynamic theory of its motions, and if his theory had 
included such an error as the one in question ; then his theory would have been 
fundamentally erroneous. But such was not at all what he professed to do. He 
proposed to explain certain large and very remarkable phenomena of the observed 
winds. This he did well, and in doing so he made a very important advance in 
development of true theory in respect to atmospheric motions. 
I have touched in some detail on these matters, because 1 think that remarks 
making inadequate recognition of the importance of Hadley’s true discoveries have 
sometimes been put forward in our own times. 
During a period of more than a century from the time of the promulgation of 
Hadley’s theory, in 1735, there was, I consider, little if any remarkable progress 
made in development of new speculations for better or for worse in respect to the 
grand or perennial currents of atmospheric circulation. A long time elapsed, in which 
there seems to have been little or no vigorous spirit of investigation into the signi¬ 
ficances or the relative merits of the speculations which had been propounded, or of 
effort to amend the existing theories, or to discover new truths on the subject. In 
confirmation of this it may be noticed that we find that 58 years after the publication 
of Hadley’s paper, Dalton arrived independently at substantially the same theory 
as that part of Hadley’s which dealt with the equatorial Trade Winds, and in his 
book entitled “ Meteorological Observations and Essays,”* which in 1793 he was 
preparing for publication, he gave an account of his theory, supposing it to be 
original, but he discovered, before the book was issued to the public, that he had 
been completely anticipated by Hadley’s paper, of the existence of which he had not 
been previously aware. In his preface to that book, after making recognition of 
Hadley’s priority, he goes on to say :—“ I cannot help observing here, that the 
following fact appears to be one of the most remarkable that the history of the 
progress of natural philosophy could furnish.—Dr. Halley published in the ‘ Philo¬ 
sophical Transactions ’ a theory of the trade-winds which was quite inadequate and 
immechanical, as will be shown, and yet the same has been almost universally 
adopted; at least I could name several modern productions of great repute in 
which it is found and do not know of one that contains any other.” . . . “ On the 
other hand G. Hadley, Esq., published in a subsequent volume of the said 
‘ Transactions ’ a rational and satisfactory explanation of the trade-winds, but where 
else shall we find it ?” 
It is right hereto remark further that Dalton in his own speculations did not touch 
at all upon the prevalence of west winds in extra-tropical regions, either as to its 
explanation or even as to its existence : and that he does not seem to have noticed or 
appreciated the great importance of Hadley’s theory in this respect. 
* “Meteorological Observations and Essays,” by John Dalton, D.C.L., F.R.S., 1793. Of this work 
there is also a second edition, which is a verbatim reprint issued by Dr. Dalton himself, in 1S34. 
