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 inequalities 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 1/306th of a degree of Fahrenheit.[42]--_Humboldt’s Cosmos_, vol. i. NICE MEASUREMENT OF HEAT. A delicate thermometer, placed on the ground, will be affected 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 fluctuate 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 constantly by radiation would then be wholly expended in its liquefaction, on the one hand, so as to leave no radiant surplus; while, on the other, the actual temperature at its surface would undergo no diminution. The great mystery, however, is to conceive how so enormous a conflagration (if such it be) can be kept up. Every discovery in chemical science here leaves us completely at a loss, or rather seems to remove further the prospect of probable explanation. If conjecture might be hazarded, we should look rather to the known possibility of an indefinite generation of heat by friction, or to its excitement by the electric discharge, than to any combustion of ponderable fuel, whether solid or gaseous, for the origin of the solar radiation.--_Outlines._[43] DISTINCTIONS OF HEAT.