1858.] and the Figure of the Earth. 207 



have made mistakes in the arithmetic, or the data regarding the 

 heights of the different parts of the mass may be wrong. But 

 there is the calculation, open to inspection, examiuation, and correc- 

 tion. I can only say, that when I arrived at my result (in 1854) 

 I was very much disappointed that it did not tally with the Great 

 Survey : and I tried every method (see articles 44 to 46 of my 

 paper) to make it do so: but could not succeed. And even now, 

 if no counteracting cause can be discovered to nullify the effect of 

 attraction, I should be very glad, for the sake of saving trouble to 

 the Survey operations, if the amount I arrive at could be shown to 

 be wrong. But it must be, not by the application of tests based 

 upon gratuitous assumptions, nor by any ulterior difficulties which 

 the large amount of attraction may appear to lead to ; but by an 

 examination of the calculation itself, showing that the data of 

 heights are so much out, or the arithmetical operations so far 

 erroneous, as materially to affect the result. 



8. One of Mr. Tennant's calculations in his second paper serves 

 to show the necessity of calculating and allowing for mountain 

 attraction. The diagram in the next page will illustrate this. 

 A B C is the actual arc of the meridian running through the three 

 stations Kuliana, Kulianpur, and Damargida. Aa, Bb, Co are the 

 normals to this arc at those three places, and therefore the direc- 

 tions in which the plumb-line would hang were there no disturbing 

 cause. A disturbing cause exists in the enormous mass II of the 

 Himalayan mountains which attracts the bob of the plumb-line so 

 as to make it hang in the lines aA, b'B, c'C making the angles of 

 deflexion aAa, bBb', cCc' . These angles are smaller the further re- 

 moved the station is from H. The spirit-level, the levelling of the 

 astronomical instruments — every thing regulated by gravity — is 

 affected by this disturbing cause. And the cause, owing to the 

 enormous mass of attracting matter, has its influence, unlike other 

 local and minor disturbing causes, along the whole line of the arc 

 though in a diminishing degree. This the calculation shows. 



If we were to proceed from C and move northwards, laying down 

 a horizontal line by means of a spirit-level (as in laying down a 

 base-line), we should find ourselves gradually rising above the arc 

 CBA; we should be obliged to stilt up the spirit-level, till over B 



