662 
PROFESSOR JAMES THOMSON ON THE 
The really important idea which it appears to me is suggested in this passage, is 
that in respect to the Earth’s rotation round its axis the sum of all the forward 
turning-force-influences applied by the winds to the surface of the Earth, land and sea 
included, must be equal to the sum of all the backward turning-force-influences like¬ 
wise ajoplied to the Earth’s surface; so that these force influences may be such as 
conjointly to produce no acceleration or retardation in the revolution of the Earth 
round its axis. 
In putting forward this idea he was doubtless assuming as a principle that we are 
not to attribute to the thermal influence of the Sun any effects in altering the rotation 
of the Earth by producing winds blowing upon the Earth more effectually on the whole 
forward than backward, or the reverse. He did not, nor probably did anyone else till 
long after his time, notice the now known principle that the Sun and Moon can, by 
their attractions, apply' to fluids on the Earth—to the sea or to the atmosphere— 
turning forces* which these fluids must communicate to the solid earth, and which 
must, in very long periods of time, make changes on the Earth's rotation. Such 
influences, however, are certainly so very small comparatively to those Hadley had 
under consideration as occurring in the action of the equatorial Trade Winds from the 
east, and the winds of higher latitudes from the west, that his not knowing of them 
is not to be regarded as derogating from the practical or substantial truth and 
validity of his Theory of the Winds in its main features. 
In the account I have given of Hadley’s theory of -the primarily important 
perennial features of atmospheric circulation, I have endeavoured faithfully to give a 
fair and favourable account of the truths which he brought to linlit, I have not held 
o O 
it as a duty to bring under review every statement or phrase to which objection might 
be taken by an adverse critic. There is one mistake, however, into which Hadley 
fell, and which is too important to be passed over without notice. This error, 
although incorporated by himself along with his true explanations in respect to the 
causes of the equatorial Trade Winds and of prevalent westerly winds of higher 
latitudes is quite separable from those true explanations; and its elimination does 
not make any break down in any essential part of his reasoning as to the real condi¬ 
tions of the atmospheric motions. His error pertained not to his suppositions as to 
the actual motions of the real air, but to supposed motions and behaviour of air in an 
ideal case which he adduced as a simplified illustration intended to be helpful to the 
consideration of the more complex conditions of the real case. The two cases—the 
ideal and the real—are not explicitly and distinctively specified by himself, but they 
are brought implicitly under consideration in his statements to the following effect :— 
Firstly .—That air having been in an approximately calm condition at one of the 
Tropic Circles, and having moved thence in the Trade Wind to the Equator, will, on 
arriving at the Equator, retain still the same absolute eastward velocity that it had 
'* Any system of forces which can be balanced by what nnder the nomenclature of Poinsot is called a 
couple, may be described as a turning-force-influence, and may now with advantage be called a torque. 
