IMPROVEMENT of TIME-PIECES. 27 



are removed, is that arifing from the unequal denfity of 

 the air, which by varying the a£lual weight of the pen- 

 dulum will accelerate or retard its motion 1 he effeds 

 arifmg from this caufe will indeed be found very Imal!, 

 for if we fuppofe the greateft range of the barometer to 

 be three inches, which indicates a change of denfity in 

 the air of about one tenth of the whole ; and fuppofmg 

 lead, of which pendulums are generally made, to be 

 8,800 times heavier than air, the variations of the adlual 

 weight of a pendulum may be one-8 8000th part of its 

 whole weight, and confequently the change in its rate of 

 going one-i 76000th part. And, as there are 86,400 

 feconds in a day, the clock may vary in its rate of going, 

 from this caufe, about ^ a fecond in 24 hours. Men- 

 tioning the barometer feems naturally to point out a 

 remedy for this caufe of irregularity by means of that in- 

 ftrument. But my defign is at prefent to defcribe a very 

 different and extremely fimple method, which though only 

 a matter of curiofity at prefent, may at lome future time 

 perhaps be found ufetul ; efpecially as the variation above 

 mentioned is governed folely by the atlual denfity of the 

 furrounding air, and the barometer can only give the 

 weight of an intire column, which does not flridlly 

 eorrefpond wuth the denfity of its bafe ; whereas the 

 method I propofe depends on the real denfity of the air 

 furrounding the pendulum, and nothing elfe. 



Let AIS (Plate 1. Fig. ©.) be a pendulum vibrating on the 

 point A, and removed from the perpendicular line DE. Let 

 the inflexible rod be continued from BA to C, and let a 

 body C, of equal dimenfions with the pendulum B, but 

 hollow and light as poflible, be fixed on the rod, making 

 AC equal to AB. Now it is evident that B will be preffed 

 upwards by a force equal to the weight of its bulk in air, 

 and its defcent retarded . But the body C, will be equally 

 preffed upwards, and confequently the motion of the 



pendulum 



