186 Things not generally Known. 



of 



THE LENGTH OF THE DAY AND THE HEAT OF THE EAKTH. 



As we may judge of the uniformity of temperature from the 

 unaltered time of vibration of a pendulum, so we may also 

 learn from the unaltered rotatory velocity of the earth the 

 amount of stability in the mean temperature of our globe. 

 This is the result of one of the most brilliant applications of 

 the knowledge we had long possessed of the movement of the 

 heavens to the thermic condition of our planet. The rotatory 

 velocity of the earth depends on its volume ; and since, by the 

 gradual cooling of the mass by radiation, the axis of rotation 

 would become shorter, the rotatory velocity would necessarily 

 increase, and the length of the day diminish with a decrease of 

 the temperature. From the comparison of the secular inequa- 

 lities in the motions of the moon with the eclipses observed in 

 former ages, it follows that, since the time of Hipparchus, that 

 is, for full 2000 years, the length of the day has certainly not 

 diminished by the hundredth part of a second. The decrease 

 of the mean heat of the globe during a period of 2000 years has 

 not therefore, taking the extremest limits, diminished as much 

 as s^th of a degree of Fahrenheit.* Humboldtfs Cosmos, vol. i. 



NICE MEASUREMENT OF HEAT. 



A delicate thermometer, placed on the ground, will be af- 

 fected by the passage of a single cloud across a clear sky ; and 

 if a succession of clouds pass over, with intervals of clear sky 

 between them, such an instrument has been observed to fluc- 

 tuate accordingly, rising with each passing mass of vapour, and 

 falling again when the radiation becomes unrestrained. 



EXPENDITURE OF HEAT BY THE SUN. 



Sir John Herschel estimates the total Expenditure of Heat 

 by the Sun in a given time, by supposing a cylinder of ice 45 

 miles in diameter to be continually darted into the sun with 

 the velocity of light, and that the water produced by its fusion 

 were continually carried off : the heat now given off con- 



* This fraction rests on the assumption that the dilatation of the suhstances 

 of which the earth is composed is equal to that of glass, that is to say, 1S * 00 for 

 1. Regarding this hypothesis, see Arago, in the Annuaire for 1834, pp. 177-190. 



