136 EFFECTS OF GRAVITY ON TIMEPIECES. 



Experiments which vibrated seconds at Paris, required to be shortened a 

 dulum dcTnot ^ ne anc * a 9 uarter at Cayenne, four degrees from the equa-r 

 agree. tor. Mr. Couplet the younger observed, that a pendulum, 



which swings seconds at Paris, must be shortened 2§ lines 

 somewhere in Portugal, which is more than Sir Isaac New- 

 ton allowed from the pole to the equator. Messrs Picart 

 and de la Hire found the length of the pendulum, which 

 beats seconds, the same at Bayopne, at Paris, and at Urani- 

 bourg in Denmark. Mr. Cassini pretended to prove from 

 experiments, that the polar diameter is the longest. In short, 

 conclusions can hardly differ more widely. 



(f) It may be thought, that a decrease of gravity will not 

 have the same effect as lessening the weight of a balance. I 

 suppose gravity constitutes weight. They decrease and va~ 

 nish together. It is the inertia of the balance, that regu- 

 lates the vibrations of the spring; but its inertia arises from 

 its weight. They decrease and vanish together. I do not 

 mean however to assert, that there is no variation of gravity 

 in different latitudes. I only suspect, that it is considerably 

 ]e6s than the received opinion allots to it, and that it is pro-? 

 bably irregular. The centrifugal force about the equator is 

 a principle evidently tending to lessen the effect of gravity; 

 but the internal structure of the Earth, of which we know 

 nothing, may lessen or vary its effect. 

 Experiments {g) Those who have inclination and opportunity to make 



with the pen- expe ritnents or observations on this subject will have no oc- 



dulum and ba- r ■ ■ J 



lance may be casion in the first instance to go any great distance. Two 



degrees about this latitude, according to the table, produce 

 not less than 7§" per day of errour in the pendulum; and 

 if the balance be found as accurate a measure of the varia- 

 tion of gravity as the pendulum by operating the contrary 

 way, there will arise a difference between them of 15" per 

 day; a quantity very discernible. Many watches are to be 

 found, that do not vary in their daily rate so much as 7f" 

 for a considerable time, and clocks not nearly so much. 

 Portable pen- (A) A pendulum rod might be made more portable, by 

 dulum rod in ^ e i n g ' m three pieces to join together with perfect accuracy, 

 and consequently not subject to be bent, by which the 

 length would be altered; and the ball being also fixed fast, 

 and the time of the vibration ascertained in some latitude, 



it 



made within 

 150 miles. 



different 

 pieces. 



