100 Mr. J. Croll on Ocean- currents in relation to 



The result at which he arrived is this : that, were the surface of 

 the globe all water, 71°'7 would be the temperature of the equator 

 and 12°" 5 the temperature of the poles ; and were the surface all 

 land, 109 o, 8 would be the temperature of the equator and — 25°'6 

 the temperature of the poles. 



But in Professor Forbes's calculations no account whatever 

 is taken of the influence of currents, whether of water or of 

 air, and the difference of temperature is attributed wholly to 

 difference of latitude and the physical properties of land and 

 water in relation to their powers in absorbing and detaining the 

 sun's rays, and to the laws of conduction and of convection which 

 regulate the internal motion of heat in the one and in the other. 

 He considers that the effects of currents are all compensatory. 



" If a current of hot water/' he says, " moderates the cold of 

 a Lapland winter, the countercurrent, which brings the cold of 

 Greenland to the shores of the United States, in a great measure 

 restores the balance of temperature, so far as it is disturbed by 

 this particular influence. The prevalent winds, in like manner, 

 including the trade- winds, though they render some portions of 

 continents, on the average, hotter or colder than others, pro- 

 duce just the contrary effect elsewhere. Each continent, if it 

 has a cold eastern shore, has likewise a warm western one ; and 

 even local winds have for the most part established laws of 

 compensation. In a given parallel of latitude all these secon- 

 dary causes of local climate may be imagined to be mutually 

 compensatory, and the outstanding gradation of mean or normal 

 temperature will mainly depend, 1st, upon the effect of lati- 

 tude simply, 2nd, on the distribution of land and water con- 

 sidered in their primary ox statical effect/' 



It is singular that a physicist so acute as Professor Forbes 

 should, in a question such as this, leave out of account the in- 

 fluence of currents, under the impression that their effects were 

 compensatory. 



If there is a constant transference of hot water from the equa- 

 torial regions to the polar, and of cold water from the polar re- 

 gions to the equatorial (a thing which Professor Forbes admitted), 

 then there can only be one place between the equator and the 

 pole where the two sets of currents compensate each other. At 

 all places on the equatorial side of this point a cooling effect is 

 the result. Starting from this neutral point, the preponderance 

 of the cooling effect over the heating increases as we approach 

 towards the equator, and the preponderance of the heating 

 effect over the cooling increases as we recede from this point 

 towards the pole — the cooling effect reaching a maximum at the 

 equator, and the heating effect a maximum at the pole. 



Had Professor Forbes observed this important fact, he would 



