403 Archdeacon Pratt on the Fiyure of 



as above. (2) By pendulum experiments made in many parts 

 of the earth it is 2 to*^- (^) I'l'om the effect of the protuberant 

 parts of the earth's mass on the motion of the moon in latitude 

 and longitude, Laplace made the depression very slightly less 

 than 3^oth. (4) By the measurement of arcs of meridian in 

 different parts of the world and observing the latitudes of their 

 extremities, and comparing arcs in high latitudes with arcs in 

 low latitudes (which has always been considered necessary to 

 Uminate certain eri'ors of observation), the depression has 

 3en found to be slightly less than ^^^gth of the equatorial 

 radius. 



3. But it will be observed that these methods all regard the 

 earth as a whole : they do not determine anything regarding the 

 separate parts. Indeed, as I have already intimated, it has long 

 been acknowledged that the fourth method fails, when arcs 

 near each other, and therefore appertaining to one portion only 

 of the earth's surface, ai'e compared. The fact is, that phy- 

 sical and practical geology teach us, that the surface of the earth 

 has for ages been undergoing changes of level in such a way as 

 most materially to disturb the exact spheroidal form. The thick- 

 ness of the crust, which Mr. Hopkins in his 'Physical Researches' 

 shows to be very great, is a proof that ages must have elapsed 

 since the earth's form ceased to be affected by hydrostatic prin- 

 ciples. Sufficient time has therefore elapsed for the exact sphe- 

 roidal form to be pretty nearly obliterated. The spheroid (of 

 3^^oth depression) is the average spheroid, or the spheroid which 

 more nearly represents the earth's irregular form than any 

 other — some parts being slightly above it, and some slightly 

 below it. 



4. It is therefore perfectly gratuitous to assume, as has gene- 

 rally been done, that the form of the earth is near an exact sphe- 

 roid; that is, that the meridians are all equal ellipses, and 

 the arcs of longitude circular. But this is assumed in Mr. Ten- 

 naut's examination of my results. I show that, if no compensa- 

 ting cause sufficient to counteract the attraction of the Hima- 

 layas can be found, the ellipticity of the ellipse most nearly 

 representing the Indian arc between Kaliana (north lat. 29° 30' 

 48") and Damargida (north lat. 18° 3' 15") is -^^j;th, and not 

 the mean value ^^^ . Mr. Tennant's object is to test this. But 

 how does he test it ? Not by examining my calculation, but 

 by comparing my result with the curvature of other arcs : (1) 

 with the prolongation of the arc down to Punnse (north lat, 

 8° 9' 32"), and (2) with an arc of longitude running west from 

 Kalianpur to Karachi. The only question, therefore, which he 

 can solve is, M^hether his arcs and mine belong to one and the 

 same spheroid or not ; not whether my calculation is right or 



