1S59.] Reply to Mr. Prates letter to the Asiatic Journal. 19 



an ellipse strictly analogous to that in common use in physical as- 

 tronomy and known as the orbit due to the varied elements ; where- 

 as the mean or fundamental ellipse derived from a number of dis- 

 tant observations will nearly, though not quite, satisfy all and bear 

 the same relation to the previous one that the mean orbit does to 

 the varied orbit. 



7. Here the analogy ceases. The Heavenly bodies in connection 

 with the sun are few and definite ; we can thus assign the law of 

 variation of the elements of the orbits. On the earth we cannot, 

 and in the heavens as on the earth the varied elements without this 

 law, are useless. The mean elements in the heavens w T ould give 

 places not very far removed from observation for a limited time, on 

 the earth they would do so always, the difference arising from the 

 motion inter se of the celestial disturbing masses and their fixity on 

 the earth. 



8. I have now I think sufficiently explained my reason for con- 

 sidering (from theory) the true form of the earth to be an absolute 

 ellipsoid. I now proceed to consider Mr. Pratt's 3rd and 4th paras. 

 — Arguing from the known changes in the form of the apparent 

 surface w r hich is visible to our eyes, Mr. Pratt reasons that there are 

 changes in the curvature of the unperturbed surface, by which I 

 mean, that which is the result of removing entirely the effects of all 

 irregularities. Nothing could I think possibly be more fallacious 

 than the argument. The change of outward form of the earth is 

 caused by the transference of matter, and it is known principally by 

 relative changes in the height of adjacent parts. In consequence of 

 these changes, are also changes in the perturbed form of the earth, 

 but when we eliminate all the effects of disturbing matter we 

 evidently must, both before and after the changes, attain the iden- 

 tically same fundamental form. 



9. Indeed we only know of these so-called actual changes except 

 in very exceptional cases* (which moreover are of small extent) 

 by reference to the sea level whose changes (being those of the 

 disturbed surface) are small in comparison, if the fundamental form 

 be fixed. If that, however, be liable to variation we have lost our 

 reference, and we cease to have evidence of the changes of height. 



* Such as the volcanic peak Jorulco and Coral Isles, and landslips. 



D 2 



