VI 



APPENDIX. 



of which E represents the earth and M the moon. Now it first 

 becomes convenient to ascertain the periodic time of a particle, or 



Pig. 2. 



of an inappreciable mass, revolving at the mean distance of the 

 moon ; and this may be ascertained by means of the formula for 

 the periodic time which we designate as (T) ; M and m being the 

 masses in question ; and we shall have 



(T) = 



rtr-f 



VM + m 



Then when m is insensible, 



and, when r is the same for both, 



2rtrf 



(T) ~ n/M 



whence 



* Encyclopaedia Metropolitana, Physical Astronomy, Sect. v. 

 (24) 



