64 HISTORY of the SOCIETY. 



Dr Stewart * n *^ e power of pure Geometry. His defign undoubtedly was, 

 to purfue that method of approximation which Sir ISAAC 

 NEWTON had begun, and which CALLENDRINI, MACHIN and 

 WALMSLEY had greatly improved; and, by ufing the methods 

 of Geometry, he hoped to reduce the problem to its ultimate 

 fimplicity. Such an undertaking was worthy of a great Geo- 

 meter, and of a Philofopher, who confidered that one of the 

 chief obftruclions to the advancement of knowledge, is the 

 difficulty of fimplifying that knowledge, which has already 

 been acquired. We muft regret, therefore, that the decline of 

 Dr STEWART'S health, which began foon after the publication 

 of the Tracts, did not permit him to purfue this investigation. 

 THE other objecT: of the Tracts was to determine the diftance 

 of the fun, from his effect in difturbing the motions of the 

 moon. The approach of the tranfit of Venus, which was to 

 happen in 1761, had turned the attention of Mathematicians 

 to the folution of this curious problem. But when it was 

 confidered, of how delicate a nature the obfervations were from 

 which that folution was to be deduced, and to how many acci- 

 dents they were expofed, it was natural, "'that fome attempt 

 fhould be made to afcertain the dimenfions of our fyftem, by 

 means lefs fubjecl to difappointment. Such accordingly was 

 the defign of Dr STEWART; and his enquiries into the lunar 

 irregularities had furnifhed him with the means of accom- 

 plifhing it. 



THE theory of the compofition and refolution of forces 

 enables us to determine what part of the folar force is employ- 

 ed in difturbing the motions of the moon ; and, therefore, 

 could we meafure the inftantaneous effect of that force, or the 

 number of feet by which it accelerates or retards the moon's 

 motion in a fecond, we fhould be able to determine how 

 many feet the whole force of the fun would make a body, 

 at the diftance of the moon, or of the earth, defcend in a fe- 

 cond, and, confequently, how rmich the earth is, in every in- 



ftant, 



