18 Reply to Mr. Pratt's letter to the Asiatic Journal. [No. 1 . 



is defined by tho mean sea level ; and ifc is on tiiis consequently that 

 our arcs are measured. As this differs from the fundamental ellip- 

 soid, only in consequence of the attractions of the irregularities, it 

 follows, that, could we remove all the consequences of the attrac- 

 tions of all the irregularities, we should have the lengths and 

 amplitudes of our arcs as though they were measured on the fun- 

 damental undisturbed ellipsoid. Every difference is caused by some 

 irregularity ; and this is certain, though we may be unable to assign its 

 origin. That we shall ever be able to account for all these differences 

 I do not expect. Omniscience alone could assign the places and masses 

 of all the portions of the earth — but, that irregularities sufficient to 

 account entirely for every deviation from the fundamental form 

 exist appears a result of the laws of matter absolutely unquestionable. 

 5. The case of the form of the earth is thus assimilated to that 

 of the orbit of a planet. Did the solar centre of our system alone 

 exist with one planet, this last body would describe a rigorous ellipse 

 about the sun ; but, in consequence of the existence of other pla- 

 nets, this mean ellipse nowhere satisfies observation, but we can al- 

 ways find an ellipse which will correspond to three observations but 

 will soon exhibit sensible deviations from observation. It is univer- 

 sally recognized that this is a legitimate consequence of the law of 

 gravitation. No one for many years has ventured to doubt that the 

 apparent irregularity of a planet's motion is caused by the attrac- 

 tion of other planets. Theory has fully accounted for all the devi_ 

 ations ; and lately, when the observed positions of the planet 

 Uranus were found undoubtedly to differ from the computed places 

 after all known corrections were applied, Astronomers did not assert 

 that the orbit was nearly but not quite an ellipse, that the law of 

 change could not be assigned, &c. ; on the contrary, the develop- 

 ment of the irregularity brought about a general tendency to seek 

 its cause, and finally, Messrs. Adams and Leverrier predicted within 

 narrow limits, the place and mass of the disturbing planet which we 

 now call Neptune. Had they followed the course Mr. Pratt seems 

 to take, they would have upheld the irregularity of the orbit ; and, 

 what is now a triumph for the universal law of gravitation, would 

 have been a reproach to science. 



G. The uncorrected data of any two arcs close together will give 



