128 Mr. J. Croll on the Change in the Obliquity of the Ecliptic, 



than the proportion of those which, having emerged through 

 the first plate, can pass through the second." I have always 

 understood that those researches prove the very reverse, viz. that 

 the percentage of heat transmitted through the first plate is 

 less than that through the second. 



I presume it may now be regarded as established that when a 

 beam of light or of heat passes through any substance, those 

 rays are cut off which agree m period with those of the molecules 

 of the substance through which the beam passes, and that the 

 principal part of the absorption takes place on the beam entering 

 the substance, and that, when all the rays which agree in period 

 are cut off (when the beam is " sifted"), those rays which re- 

 main pass onwards with but little obstruction. Then, if this 

 beautiful theory be correct, it necessarily follows that all Tables 

 calculated upon the principle that the amount of the sun's heat 

 cut off in passing through the atmosphere is proportionate to 

 the number of aerial particles that the rays have to encounter 

 before reaching the surface of the earth must be (< wholly erro- 

 neous." 



But supposing it were the case that the annual amount of 

 heat reaching the poles to that reaching the equator is actually 

 no more than as 1 to 8, so much the better is it for my argu- 

 ment; for it would show that it is to ocean-currents, and not 

 to the direct heat of the sun, that the polar regions are chiefly 

 indebted for their temperature. If the absolute quantity of the 

 sun's heat reaching the poles be only J of that reaching the 

 equator, then why is there only a difference of about 80° between 

 the temperature of the poles and that of the equator ? Accord- 

 ing to Mr. Moore's own calculations (Lyell's ' Principles/ vol. i. 

 p. 294, new edition) there ought to be a difference of from 

 200° to 300°. Why, then, is not the mean annual temperature 

 of the poles somewhere between 100° and 200° below zero, in- 

 stead of at zero as at present ? This comparatively high tem- 

 perature of the polar regions must in such a case be chiefly at- 

 tributed to the influence of ocean-currents ; and if so, then to 

 what an enormous extent would the climate of the arctic regions 

 be affected by even a very slight deflection of the equatorial cur- 

 rent of the Atlantic into the Southern Ocean, occasioned by a 

 change in the excentricity of the earth's orbit. 



