DOG DAYS. 



tation reaches its maximum ; but the temperature of the weather 

 does not therefore cease to increase. After the 21st of June, there 

 continues to be still a daily augmentation of heat, for the sun still 

 continues to impart more heat during the day than is lost during 

 the night. The temperature of the weather will therefore only 

 cease to increase when, by the diminished length of the day, the 

 increased length of the night, and the diminished meridional 

 altitude of the sun, the heat imparted during the day is just 

 balanced by the heat lost during the night. There will be, then, 

 no further increase of temperature, and the heat of the weather 

 will have attained its maximum. 



But it might occur to a superficial observer, that this reasoning 

 would lead to the conclusion that the weather would continue to 

 increase in its temperature, until the length of the days would 

 become equal to the length of the nights ; and such would be the 

 case, if the loss of heat per hour during the night were equal to 

 the gain of heat per hour during the day. But such is not the 

 case ; the loss is more rapid than the gain, and the consequence is, 

 that the hottest day usually comes within the month of July, but 

 always long before the day of the autumnal equinox. 



The same reasoning will explain why the coldest weather does 

 not usually occur on the 21st of December, when the day is 

 shortest and the night longest, and when the sun attains the 

 lowest meridional altitude. The decrease of the temperature 

 of the weather depends upon the loss of heat during the night 

 being greater than the gain during the day ; and until, by the 

 increased length of the day and the diminished length of the 

 night, these effects are balanced, the coldest weather will not be 

 attained. 



These observations must be understood as applying only so far 

 as the temperature of the weather is affected by the sun, and by 

 the length of the days and nights. There are a variety of other 

 local and geographical causes which interfere with these effects, 

 and vary them at different times and places. 



69. Since the sun moves through one-half of the circumference 

 of the heavens between the 20th of March and the 23rd of Sep- 

 tember, and through the other half between the 23rd of September 

 and the 20th of March, in each half-year moving over 180 

 of the ecliptic (the name given to the apparent course of the 

 sun over the firmament), it might be inferred that these two 

 intervals must necessarily be equal. But if we take account of 

 the days included in them respectively, we shall find that such is 

 not the case. 



The numbers of days in the two intervals in 1854, for example, 

 were : 



39 



