On the Improvement ofTime^Ketpers, 290 



or retard its motion. The effects arising from this cause will 

 indeed be found very small ; for, if we suppose the greatest 

 range of the barometer to be three inches, which indicates 

 a change of density in the air of above one-tenth of the 

 whole 'y and supposing lead, of which pendulums are gene- 

 rally made, to be 8,800 times heavier than air, the variations 

 of the actual weight of a pendulum may be l-68000th part 

 of its whole weight, and consequently the change in its rate 

 of going 1-1 76000th part. And, as there are 86,400 seconds 

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

 cause, about half a second in 24 hours. Mentioning the ba- 

 rometer, seems naturally to point out a remedy for this causa 

 of irregularity by means of that instrument. But my design 

 is at present to describe a very different and extremely simple 

 method, which, though only a matter of curiosity at present, 

 may at some future time, perhaps, be found useful; especially 

 as the variation above mentioned is governed solely by the 

 actual density of the surrounding air, and the barometer can 

 only give the weight of an entire column, which does not 

 strictly correspond with the density of its base 3 whereas the 

 method I propose depends on the real density of the air sur- 

 rounding the pendulum, and nothing else. 



Let AB (Plate VII. fig. 1.) 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 dimensions with the pendulum B, but 

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

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

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

 its descent retarded. But the body C will be equally pressed 

 upwards, and consequently the motion of the pendulum to* 

 wards the perpendicular will be as much accelerated. These 

 two forces, therefore, will destroy each other, and the pen- 

 dulum will perform its vibrations in equal times, whether the 

 air be light or heavy, dense or rare. 



I have, for greater perspicuity, described the most simple 

 case, but perhaps not the most eligible; for if we can enlarge 

 the vessel or body C in any proportion, the distance of its 

 centre from A may be diminished at the same rate. 



P p 2 However 



