280 LIGHT SCIENCE FOR LEISURE HOURS. 



tions of the variation of mean diurnal temperature, 

 since it is clear that a cause of variation due to objects 

 external to the earth could affect only the temperature 

 of certain hours of one day or of several days. A cluster 

 of meteors between the earth and the sun might 

 diminish the mid-day heat ; one external to the earth's 

 orbit might increase the nocturnal temperature ; and 

 though in either case the mean diurnal temperature 

 would be affected, yet it is obvious that the effect 

 would be masked in taking the mean, or even that two 

 or more opposing influences might cancel each other. 

 If it could be shown that the curve for mid-day, or for 

 midnight heat corresponded to the curve of mean heat, 

 Ertel's theory would be overthrown at once ; since, for 

 its support it is necessary to show that depressions in 

 the mean curve are due to mid-day loss of heat, and 

 elevations to midnight gain of heat. 



There are, however, terrestrial causes to which the 

 irregularities of our curve (which irregularities, be it 

 remembered, represent regularly recurring irregulari- 

 ties of heat) may be ascribed. For instance, there can 

 be no doubt that our climate is considerably affected by 

 the changes which take place in the Polar seas ; and it 

 may not unfairly be assumed that the processes by 

 which different regions of Polar ice are successively 

 set adrift (to be carried southward by the strong under- 

 current known to exist in the northern Atlantic Ocean), 

 take place at epochs which recur with tolerable regu- 

 larity. And it may be that the irregularity of the 

 rising as compared with the falling of the heat-curve 



