40 



earth's crust is known not to be homogeneous, experiments 

 have been made by Maskelyne, Cavendish, &c, for deter- 

 mining its force of gravity ; which prove the average density 

 to be 5 # 4, the whole mass being double that of the crust. 

 It is known that the excess of the equatorial over the polar 

 radius, divided by the latter, would be one in 230, if the earth 

 were homogeneous throughout, all the particles attracting each 

 other, while it would be one in 580 if the force of attraction 

 acted solely at the centre of the mass, or nearly as 289*44 to 

 308*27. In applying these remarks for obtaining a true 

 parallax at the coming transit of Venus, there is the following 

 dimculty,for all observations made on the plane of the meridian, 

 the perpendicular, or line pointing to the zenith, does not 

 coincide with the line directed from the earth's centre through 

 the place of observation, except at the equator and the 

 poles. 



Eespecting the figure of the earth as a whole, says the 

 late Sir John Herschel, it may be considered as spherical ; the 

 diameter coinciding with the axis is about -^th part shorter 

 than the equatorial circle, which is, says the same authority, 

 so trifling that if a model of such proportions were turned 

 in wood, the nicest eye would not detect the flattening, since 

 in the diameter of a globe of 15 inches, the difference would 

 be only one-twentieth of an inch, and with the means 

 employed for obtaining the true figure of the earth, any error 

 in observation, with proper care, in measuring its true 

 meridional value, can hardly exceed half a second. Any sus- 

 picion then that the above quoted idea of a double ellipticity 

 is only an approximation to truth, and may have an important 

 bearing in ascertaining the sun's distance, is much lessened 

 when it is known that the discrepancy is not so great as at 

 first sight may be supposed,and that it may entirely disappear 

 in working out this great problem at the forthcoming transit. 



The application of the transit of Venus to the determination 

 of the sun's distance, is not an easy subject, from the enor- 

 mous distance of the sun, when compared with the smallness 

 of our own globe, for a base line ; if it was otherwise the 

 parallax question would be a simple matter. Mr. Procter 

 (whose beautiful sun- views of the earth, drawn to illustrate 

 the seasons, shew changes similar to those of sun-spot curves, 

 irrespective of the sun's hemisphere) remarks : — 



" During a December transit, the earth which is supposed 

 to be seen from the sun, moving from right to left, with 

 rotation shifting points on her surface ; the shifting due to 

 this cause, is greatest on the Equator, which is adverse to 

 that of the earth's motion of rotation, except at stations 

 in high latitudes, where Venus transits witfi. the excess of 



