174 PHILOSOPHICAL TRANSACTIONS. [aNMO 1704. 



2. As in vacuo (where the pressure of the atmosphere is taken off) heavy 

 bodies descend quicker than they do in the open air ; so it may be observed, 

 that pendulums move swifter in the exhausted receiver, than in it unexhausted. 

 That heavy bodies descend quicker in vacuo, is evident from the swift descent 

 of the less heavy bodies, as cork, the down of sowthistles, the lightest feather, 

 &c. which do all precipitantly descend, like a stone, in a tall exhausted receiver^ 

 And that the pendulum, in our experiment, moved faster in vacuo, is manifest'" 

 from its vibrating but 2 seconds in an hour slower, when the vibrations were -J- 

 of an inch on a side, enlarged by the highest rarefaction of the air. Whereas 

 I find by experiment, that near the same increase of the vibrations, in the open 

 air, makes the pendulum go 6 or more seconds slower in an hour. I say nearly 

 the same increase, because it is scarcely possible to manage the pallets so, as 

 nicely to make the same vibrations as were in vacuo. 



3. I query, Whether the variations of pendulums observed under the equi- 

 noctial, and between the tropics, do not arise as much, or more, from the 

 rarity of the medium, and the increase of the vibrations consequent thereupon ? 

 It is scarcely, I think, to be doubted, but that the air is much thinner, and finer 

 near the line, than it is without the tropics. And it is evident from the baro- 

 meter, that on the tops of high mountains the atmosphere gravitates less than 

 nearer the centre. And therefore, though I like the notion of the decrease of 

 gravity from the increase of the distance from the earth's centre, yet I am apt 

 to think that this is not the only reason of the phenomenon. 



I wish therefore that Capt Halley, when he observed his clock go slower 

 at St. Helena than in England, had at the same time observed whether the 

 vibrations were not enlarged. It might be therefore very well worth the while 

 for such as have opportunity, to notice, whether their pendulums between the 

 tropics do not make larger arches, than in higher latitudes? Also in what lati- 

 tude they begin to alter? Whether the vibrations be greater nearer the line, 

 than in any other part between the tropics? Or whether the greatest increase 

 be not always in those places where the sun passes their zenith ? If the vibra- 

 tions be found larger under the line, or in any other part of the torrid zone, 

 then it may be observed how much larger they are, and in what proportion they 

 increase or decrease, by approaching nearer to, or receding from the place of 

 their greatest increase ? 



Also it may be worth observing, whether pendulums do not vary on the tops 

 of high places, or in different states of the atmosphere, according as the mer- 

 cury is high or low in the barometer ? But then in this, and indeed in the 

 former cases, it is necessary, or at least very expedient, that the movement be 

 so exactly well made, that the power, whether weight or spring, do at all times 



