EFFECTS OP GRAVITY ON TIMEPIECES. \§J 



it might be taken in pieces, and when reconstructed else- 

 where the difference of time would show the length requi- 

 site more correctly than any other sort of measurement, and 

 consequently the difference of gravity. Several little er- 

 rours would be avoided by always using the same pendulum. 



(i) Mr. Cumming, in his Elements of Clock and Watch Vibrations of a 

 Work (notes to Article 410) says, « In the pendulum, gra- ^pTndot gT. 

 «' vity is the motive force ; and in the balance the spring : vity. 

 " the vis insita is the resistance in each ; and the contrary: 

 *' therefore, when the motive force is in each as the resist- 

 " ance, the velocities and times must be equal. Hence it 

 ** also happens, that (c. p.) the balance measures the same 

 " time in all latitudes." And afterward : " Here by alter- 

 " ing the weight, is strictly meant, altering the vis inertice; 

 " for the vibrations of a balance, whose centre of gravity 

 " coincides with its centre of motion, have not the least 

 M dependence on gravitation, otherwise it would alter iti 

 *f times in different latitudes as well as the pendulum." 



(k) Here it appears to me, that Mr. Gumming considers 

 the vis insita and vis inertia? of matter, as powers wholly in- 

 dependent of its gravitation or weight. I suppose it will 

 not be doubted, that a balance becomes heavier or lighter, 

 as gravity is greater or less, and if hung by its spring, it 

 will stretch the spring proportionally to the power of gra- 

 vity: still he considers its vis insita to remain unaltered with 

 its motive force, and consequently that it will measure the 

 same time in all latitudes. 



(/) I think it is generally allowed, that the vis inertias is vis inertix. 

 proportional to the quantity of matter. We distinguish a 

 greater or less quantity of matter by its weight, consequently 

 its vis inertiae will be proportional to its weight, for it is cer- . 

 tain that any thing heavy has more resistance than a thing 

 that is lighter. 



(m) Suppose a watch vibrating with a steel balance, and, Balances of 



without any other alteration, let it be taken off and replaced dl ff erent 



J .i ' . . weights require 



with a gold balance of equal magnitude in every way, which, springs of dit- 



for convenience we shall suppose to be just double in weight. ferent 

 Though in this case nothing is altered but the weight, the in- 

 ertia is just doubled, for it will require another spring equally 

 strong, added to the former, to give it the same velocity. 



(n) Again 



