462 



THE EARTH. 



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 tempera- 

 ture 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 that 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 v 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 tem- 

 perature 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. 



On referring to the annual motion of the earth round the sun, it appears that 

 the position of the sun within the elliptic orbit of the earth is such that the 

 earth is nearest to the sun about the 1st of January, and most distant from it 

 about the 1st of July. As the calorific power of the sun's rays increases as 

 the distance from the earth diminishes, in even a higher proportion than the 

 change of distances, it might be expected that the effect of the sun in heating 

 the earth on the 1st of January would be considerably greater than on the 1st 

 of July. If this were admitted, it would follow that the annual motion of the 

 earth in its elliptic orbit would have a tendency to diminish the cold of the 

 winter in the northern hemisphere, and mitigate the heat of summer, so as to a 

 certain extent to equalize the seasons ; and on the contrary, in the southern 

 hemisphere, where the 1st of January is in the middle of summer and the 1st 

 of July the middle of winter, its effects would be to aggravate the cold in winter 

 and the heat in summer. The investigations, however, which have been made 

 in the physics of heat, have shown that that principle is governed by laws 

 which counteract such effects. Like the operation of all other physical agen- 

 cies, the sun's calorific power requires a definite time to produce a given effect, 

 and the heat received by the earth at any part of its orbit will depend con- 

 jointly on its distance from the sun and the length of time it takes to traverse 

 that portion of its orbit. In fact, it has been ascertained that the heating power 

 depends as much on the rate at which the sun changes its longitude as upon 

 the earth's distance from it. Now it happens that in consequence of the laws 

 of the planetary motions, discovered by Kepler, and explained by Newton, 

 when the earth is most remote from the sun, its velocity is least, and conse- 

 quently the hourly changes of longitude of the sun will be proportionally less. 

 Thus it appears that what the heating power loses by augmented distance, it 

 gains by diminished velocity ; and again, when the earth is nearest to the sun, 

 what it gains by diminished distance, it loses by increased speed. There is 

 thus a complete compensation produced in the heating effect of the sun by 

 the diminished velocity of the earth which accompanies its increased dis- 

 tance. 



The place of aphelion, or the point where the earth is most distant from the 

 sun, and the place of perihelion, or the point where it is nearest to the sun, are 



