GRANT) CURRENTS OP ATMOSPHERIC CIRCULATION. 
663 
when at the Tropic, and so will at the Equator have less velocity of absolute eastwai’d 
motion than the Earth there has, by 2083 miles per day, or 87 miles per hour, and 
that so it will be moving relatively to the Earth there as a wind blowing at the rate 
of 2083 miles per day from east to west. 
And Secondly :—That as an amendment on the previous statement, it is to be 
considered that “ before the air from the Tropicks can arrive at the Equator, it must 
have gained some motion eastward from the surface of the earth or sea, whereby its 
relative motion will be diminished, and in several successive circulations may be 
supposed to be reduced to the strength it is found to be of.” 
In reading these two statements conjointly we may w r ith confidence judge that the 
first of them is not meant to convey the actual truth in respect to the real behaviour 
of the atmosphere, but that it is only a theoretical utterance as to an ideal case, in 
which the frictional drag between the surface of the ocean and the atmosphere is left 
out of account, and that the second is that which is meant to convey the real truth. 
Now the important error into which he has here fallen, consists in his supposing that 
in an ideal case, in which the trade-wind air is regarded as frictionless and free from 
receiving any eastward or westward force-influences from the ocean below it, or as 
I will add, from the atmosphere immediately above it, it ought to be expected on 
arriving at the Equator, from a calm at the tropic circle, to retain the same amount of 
eastward absolute motion which it had when at the tropic. Instead of that, in the 
ideal case, if fully specified with due limitations, such as we may suppose were 
tacitly contemplated, without being fully thought out, the true averment would have 
been that the air on arriving at the Equator would have a velocity of eastward 
absolute motion less than that at the tropic, in ratio inverse of that of the distances 
of the two places respectively from the Earth’s axis. What I mean here to say, 
may, perhaps, without elaborate definitions and specifications, be tolerably well 
suggested in brief words, by saying that, in a vortex of free mobility, with circular 
motions round an axis, the velocities at different distances from the axis must be 
inversely as those distances. 
But now, in truth, the ideal case which Hadley touched upon, was quite outside 
of the scope of the real conditions of the atmospheric motions, which he professed to 
explain better than had been done in the attempts of others before his time. He had 
amply sufficient reason for his averment, to the effect that the real trade-wind air in 
its approach from the tropic to the Equator, under the influence of indraught towards 
the Equator, should be expected at each new place nearer to the Equator than the 
previous one, to have a less velocity of eastward absolute motion than the surface of 
the sea has at that new place, and that the frictional drag eastward applied to the 
air by the sea surface will only act towards assimilation of the eastward velocity of 
the air to that of the water, while still in principle, as in fact, leaving the air to go 
slower eastward than does the water—that is, to blow as a westward wind relative 
to the ocean. If what he professed to do had been to bring into notice a special 
