NATURAL PHILOSOPHY. 137 



maximum of temperature. They indicate that the daily variation of 

 temperature is probably one cause of the variation of intensity. We 

 find, further, that the horizontal force increases during the latter half of 

 the night, till 5 or 6 A. M., and then decreases till 10 A. M., where- 

 as the temperature falls steadily till 5 or 6 A. M., and after that be- 

 gins to rise. Thus, in the one case there are two maxima and two 

 minima, and in the other, only one of each. Again, while the tem- 

 perature falls in the afternoon and evening as rapidly as it rises in the 

 forenoon, the horizontal force decreases less rapidly during the former 

 period than it increases during the latter ; and, as intimated, the max- 

 imum of intensity is sometimes an hour or two later than the maximum 

 of temperature." 



Here are discrepancies between the actual and the theoretical vari- 

 ations of the horizontal force which are to be accounted for. Profes- 

 sor Norton calls them secondary variations, merely to distinguish 

 them, without intending to imply that they are of minor importance. 

 The inevitable inference from these discrepancies is, that if the dai- 

 ly variation of temperature is one cause of the daily variation of the 

 horizontal force, there must also be some other cause at work besides 

 this. It seems probable, for various reasons, that this additional 

 cause is merely some indirect effect of the variation of temperature; 

 the chief reason is, that the time of the secondary maximum of in- 

 tensity moves backwards and forwards with the time of sunrise. 

 Professor Norton then examines the subject with great minuteness by 

 means of tables and facts ; he takes up the arguments for and against 

 various causes, examines into the laws of the radiation of heat, and 

 finally says, " In view of all that has now been stated, it may be 

 confidently affirmed, that if the cause of the two anomalous facts con- 

 nected with the nocturnal loss of temperature be any meteorological 

 phenomenon, it must be the deposition of vapor from the atmosphere 

 in other forms than that of rain, and chiefly, therefore, in the form of 

 dew. Either this must be the actual cause, or it must consist in the 

 laws of the earth's cooling at night, irrespective of all atmospheric 

 influences." A little further on he states his conclusion in somewhat 

 different words. " I conclude that the cause of the nocturnal varia- 

 tions of the horizontal force must either consist in variations in the 

 amount of vapor deposited from the atmosphere, or be in some way 

 connected with the upward flow of heat below the earth's surface." 

 The author then takes up this latter alternative of the upward flow 

 of heat, and concludes that it "fails entirely to explain the unequal 

 losses of temperature at night in different seasons of the year." 



Having come to this conclusion, that the secondary variations can be 

 accounted for by nothing else than by the variation in the amount of 

 dew deposited in different seasons, and in different hours of the night, 

 and the consequent variation in the quantity of heat given out in the 

 condensation of vapor into dew, he makes a minute examination into 

 the quantity of dew that falls, and says, " I conclude, therefore, that 

 the heat evolved from the dew, or condensed vapor, that falls at night, 

 is nearly if not quite sufficient to reduce the theoretical decrease of 

 temperature due to radiation down to the amount which actually ob- 



12* 



