586 The Smithsonian Institution 



sufficient extent to be useful in extended geological investi- 

 gations does not come within the scope of the work. The 

 nature of the results arrived at may be illustrated by quoting 

 a paragraph regarding the planet Mercury. 



" For the planet Mercury, we find that the eccentricity is 

 always included between the limits 0.124923 and 0.2317185. 

 The mean motion of its perihelion is 5".4638o3 ; and it per- 

 forms a complete revolution in the heavens in 237,197 years. 

 The maximum inclination of its orbit to the fixed ecliptic of 

 1850 is 10 36' 20", and its minimum inclination is 3 47' 8" ; 

 while with respect to the invariable plane of the planetary 

 system, the limits of inclination are 9 io'4i" and 4 44' 27". 

 The mean motion of the node of Mercury's orbit on the 

 ecliptic of 1850 and on the invariable plane is in both cases 

 the same, and equal to 5". 126172, making a complete revolu- 

 tion in the interval of 252,823 years. The amount by which 

 the true place of the node can differ from its mean place on 

 the ecliptic of 1850 is equal to 30 8', while on the invariable 

 plane this limit is only 18 31'." 



A knowledge of the elements of the earth's orbit is espe- 

 cially interesting and important on account of the recent 

 attempts to establish a connection between geological phe- 

 nomena and terrestrial temperature, in so far as the latter 

 is modified by the variable eccentricity of her orbit. The 

 amount of light and heat received from the sun in the course 

 of a year depends to an important extent on the eccentricity 

 of the earth's orbit; but the distribution of the same over 

 the surface of the earth depends on the relative position of 

 the perihelion of the orbit with respect to the equinoxes, and 

 on the obliquity of the ecliptic to the equator. These ele- 

 ments are subject to great and irregular variations ; but their 

 laws can now be determined with as much precision as the 

 exigencies of science may require. A table of the eccentricity 



