152 Mr. J. Croll on Geological Time, and the probable 



a considerable extent. The direct heat of the moon, for ex- 

 ample, cannot be detected by the finest instruments which we 

 possess ; yet from 238,000 observations made at Prague during 

 ] 840-66, it would seem that the temperature is sensibly affected 

 by the mere change in the lunar perigee and inclination of the 

 moon's orbit*. 



Column VIII. gives the midwinter temperature. It is found 

 by subtracting the numbers in column VII. from 39°, the mid- 

 winter temperature. Column IX. shows the midwinter tempe- 

 rature of the centre of Scotland, on the supposition that the 

 Gulf-stream was diminished in volume in proportion to the ex- 

 centricity. In former papers it was explained how a change of 

 excentricity must affect ocean-currents f. 



I have not given a Table showing the temperature of the 

 summers at the corresponding periods. This could not well be 

 done j for there is no relation at the periods in question between 

 the intensity of the sun's heat and the temperature of the 

 summers. One is apt to suppose, without due consideration, 

 that the summers ought to be then as much warmer than they 

 are at present, as the winters were then colder than now. Sir 

 Charles Lyell, in his ' Principles/ has given a column of summer 

 temperatures calculated upon this principle. Astronomically the 

 principle is correct, but physically it is totally erroneous, and 

 calculated to convey a wrong impression regarding the whole 

 subject of geological climate. The summers at those periods, 

 instead of being much warmer than they are at present, would 

 in reality be far colder than they are now, notwithstanding 

 the great increase in the intensity of the sun's heat resulting 

 from the diminished distance of the sun. If a country is 

 free from snow and ice, then no doubt the temperature will rise 

 during summer as the intensity of the sun's heat increases; but 

 when such a country is enveloped in perpetual snow and ice, 

 the temperature of the summers will never rise much above the 

 freezing-point, no matter what the intensity of the sun's heat 

 may be. The physical reason of this was explained on a former 

 occasion J. In a country covered with ice, the direct heat of 

 the sun is often very intense, in fact scorching. It will raise 

 the temperature of the mercury in the thermometer exposed to 

 the direct rays of the sun, but it fails to heat the air. Captain 

 Scoresby, for example, when in lat. 80° 19' N., found the side 

 of his ship heated by the direct rays of the sun to about 100°, 



* See Professor C. V. Zenger's paper " On the Periodic Change of Cli- 

 mate caused by the Moon," Phil. Mag. for Jnne 1868. 

 + Phil. Mag. for August 1864 and February 1867. 

 I Phil. Mag. for February 1867. 



