656 
PROFESSOR JAMES THOMSON ON THE 
outer limits of the two Trade Winds vary in different seasons of the year, and are 
affected by casually varying conditions of the atmosphere in other parts of the world, 
and by the geographical configurations of the surrounding continents affecting them 
unequally in different parts; but, without minute exactitude, they may be regarded 
as occupying some such breadth as perhaps 25° or 30° on each side of the Equator. 
It was also found by mariners in those early times previous to the development of 
theories of atmospheric circulation, that in the great oceans, in the higher latitudes, 
outside of the trade-wind bands, west winds are prevalent in frequency and strength 
over winds in other directions. It became the practice of traders when going on a 
voyage from east to west to make their way into the trade-wind region, where they 
were sure of finding favouring breezes, and on their return voyage to get into higher 
latitudes, so as to take advantage of the prevailing west winds there. 
Until recent years no information was definitely gathered from observations or 
otherwise as to whether or not there be any prevalent general average tendency in those 
west winds to blow in their variations more towards the Pole or towards the Equator; 
and I avoid entering on any statements on the subject at the present historical stage, 
as that matter will be better associated with the subsequent progress of theories than 
with the early history. 
Fig. 1. 
A/ 
Q 
The explanations just given in words as to the chief features of the trade winds and 
of the west winds of higher latitudes may be supplemented so as to come more vividly 
before the imagination by aid of fig. 1. 
This figure is sketched without regard to the disturbing influences of continents and 
mountain ranges. It may be regarded as being suggestive of the most remarkable 
features which would probably present themselves in the winds if the surface of the 
world were all ocean, or were ocean mottled very uniformly with small islands. 
Now, to revert to the historical sketch already entered on, of speculations and 
theories as to perennially prevalent winds, and to variable winds which mani¬ 
fest perennial prevalence in special directions, the next theory to which I have 
