METEOROLOGY. 213 



heat received from the sun is diminishing; the discernible effect, it 

 is true, may be for ages totally unporceived, as so slow is the change 

 of eccentricity of the earth's orbit, that if equably continued it will be 

 upwards of 37,000 years before the orbit has become perfectly 

 circular, and the earth receives its least annual amount of heat, and 

 then the minor axis or shorter diameter will not have been lengthened 

 one-fiftieth of its present amount. WTicther the slow and impercep- 

 tible rale of this vast cycle produces real effects on the ceaseless 

 changes of atmospheric phenomena in all localities, we are, perhaps, 

 not in a condition to determine. 



Another effect of planetary disturbances, slightly tending in the 

 lapse of ages to affect climate, is the following : a variation in the 

 inclination of the orbits of the planets to the ecliptic, or the sun's 

 apparent annual path in the heavens, thereby affecting the apparent 

 annual excursions of the sun on each side of the celestial equator. 

 The planes of the ellipses in which the planets move ai'e differently in- 

 clined, and, from their mutual action, their inclination is subject to vari- 

 ation. In some orbits the inclination is increasing, at the same 

 time in others it is diminishing. At present the orbit of the earih 

 is becoming less' inclined to the equator (properly all these motions 

 should be referred to an imaginary fixed plane, unaffected by secular 

 variation ; but for the sake of more easy intelligibility the inclination 

 of the earth's orbit may be referred to the angle which it makes with 

 the equator). This diminution of inclination is of importance in 

 its tendency to influence the climates and seasons on the earth's 

 surface; for were this decrease of obliquity to continue until the 

 ecliptic, or sun's path, was in the line of the equator, the succession 

 of seasons would cease, and an unifonnity of temperature, so far as 

 dependant on constant equality of day and night, would in every 

 latitude ensue ; for day and night would throughout the year be of 

 equal duration. \Vlien we consider that all organic nature is 

 specially adapted to climate as at present constituted, requiring 

 annual periods of excitation and repose, it is evident that great 

 change must ensue. Again, were it possible that this varying state of 

 inclination could be gradually changed the other way, so as to be con- 

 stantly augmenting, until at length the sun's path lay on a meridian 

 of longitude, or making aright angle with the equator, the most insup- 

 portable extremes of heat and cold would in every zone be produced ; 



