212 METEOROLOGY. 



may be, in the lapse of ages, produced in the mean temperature of 

 the earth, by virtue of the mutual disturbances of the planets. This 

 power is indirect, and transmitted through the sun, tending to pro- 

 duce slight changes in his average distance, &c., &c., and is by no 

 means an immediate and direct power of planet over planet, so far as 

 temperature is concerned, that being affected solely through the 

 varying distances of the sun. The perturbation or disturbance pro- 

 duced in the motion of the planets round the sun, by their mutual 

 action on each other, tends to alter the fonn of the orbits in which 

 they severally move, changing their amount of eccentricity or ellipti- 

 city. By virtue of this mutual disturbance, the orbits of some 

 planets are becoming more circular, and "at the same time those of 

 others more elliptical. Now iha amount of eccentricity to which 

 any planet may be liable by the deiangement of its orbit is all im- 

 portant to its inhabitants ; for the amount of mean annual heat 

 received by any planet from the sun, is inversely, as the length of the 

 minor axis or shorter diameter of its orbit; so that in an oibit per- 

 fectly circular, the mean annual heat is the least possible, (the major 

 axis of course remaining, with the exception of a small and rapidly 

 periodic inequality invariable, which astronomers know to be a fact, 

 and an all important one) ; the mean annual heat, by virtue of the 

 ratio just named, increases as the orbit becomes elliptical, and the 

 minor axis consequently shortened. Now we may conceive, if the 

 mutual disturbances of the planets go on indefinitely and uncompen- 

 sated, that the state of the earth may one day, by the extreme cllipti- 

 cily of its orbit, and its consequent change of mean annual temperature, 

 become quite incompatible with the existence of present animal and 

 vegetable life. By a mathematical theorem, however, of extreme 

 beauty, it has been proved that this indefinite ellipticity is impossible, 

 and that the limits of the eccentricity of all the planets are within bounds 

 so narrow, as to preclude any important changes in their physical con- 

 dition. At the same time, from the period of the earliest observa- 

 tions to the present, the eccentricity of the earth's orbit has been 

 diminishing; and it is thought will thus continue until the earth's 

 orbit has become a circle, when the other compensating half of the 

 long cycle which these changes embrace, will take its turn ; the orbit 

 will again become slowly elliptical until its limit of eccentricity is 

 attained. From this cause, then, it appears that the mean annual 



