250 MR. THOMAS HOPKINS ON THE INFLUENCE 



on that point will have the same. Conceive such a body to 

 be urged by any impulse in the direction of a meridian 

 towards the equator. Since such impulse communicates 

 to it no increase of velocity^ it will find itself, at each 

 point of its progress, continually more and more deficient 

 in this element of movement, and will lag behind the swifter 

 surface below it, or drag upon it with a relative westerly 

 tendency. In other words, it will no longer be a direct 

 north or south wind, but, relatively to the surface over 

 which it is moving, will assume continually more and more 

 the character of a north-easterly or south-easterly one, 

 according as it approaches the equator from the north or 

 south ^' (p. 57). 



In both of these extracts it will be perceived that the 

 great departure of the tropical trade-winds from a meri- 

 dianal line is attributed to unequal rotatory velocities of 

 the surface converting the north and south winds of the 

 Atlantic to north-east and south-east winds j and in the 

 Pacific changing their direction, until over the greater por- 

 tion of its surface these become east winds, though this 

 surface is within the tropics, where comparative rotatory 

 velocity does not greatly alter. Yet all western tendency 

 of air in those parts is attributed to the air being left be- 

 hind by the rotation of the surface that is encountered. 

 Now, that air may be left behind by unequal rotating 

 surfaces is admitted. From the nature and action of phy- 

 sical force, it would seem that such must be the case. 

 But is it so left to any definite or appreciable extent? 

 The eminent writers quoted assert in general terms that 

 the retardation does take place, without thinking it neces- 

 sary to show in a scientific way what is its precise amount, 

 or what will be the difference in the retardation in lati- 

 tudes revolving at very different speeds. In short, it is as- 

 sumed, in a general way, that retardation does take place, 

 just as the ascent of air within the tropics was by Hadley, 



