60 PHILOSOPHICAL TRANSACTIONS. [aNNO 1736. 



struction of the menses, which was supposed to give rise to the diseased state 

 of the skin. 



Experiments on the Vibrations of Pendulums. By the late W. Derham, D. D. 



F.R.S. N°440, p. 201. 



The account Mr. Bradley gave in Philos. Trans. N°432, of observations made 

 at Jamaica by Mr. Campbel, with a nice pendulum-clock of Mr. Graham's 

 making, brought to mind some experiments Dr. Derham made some years be- 

 fore, which may be of use in observations of this nature. 



The first he notices are some experiments made in the year 1 704, with ex- 

 cellent instruments, on the vibrations of pendulums in vacuo ; which were 

 published in the Philos. Trans. N° 294. The sum of which is, that the vibra- 

 tions in vacuo were larger than in the open air, or receiver unexhausted : also 

 that the enlargement or diminution of the vibrations, was constantly in pro- 

 portion to the quantity of air, or rarity, or its density, left in the receiver of 

 the air-pump. And as the vibrations were larger or shorter, so the times were 

 augmented or diminished accordingly, viz. 2 seconds in an hour slower, when 

 the vibrations were largest, and less and less, as the air was re-admitted, and 

 the vibrations shortened. 



But notwithstanding the times were slower, as the vibrations were larger, yet 

 he had reason to conclude, that the pendulum really moved quicker in vacuo, 

 than in the air, because the same difference, or enlargement of the vibrations, 

 as -^ of an inch on a side, would cause the movement, instead of 2 seconds in 

 an hour, to go 6 or 7 seconds slower in the same time ; as he found by nice 

 experiments. 



The next experiments he made at several times, in 1705, 1706, and 17 12, by 

 the help of a good month-piece, that swings seconds. The weight that then 

 drove it, was about 12 or 13 pounds, and it kept time exactly by the sun's mean 

 motion : but by hanging on 6 pounds more, the vibrations were enlarged ; but 

 yet the clock gained 13 or 14 seconds in a day. 



And as the increase or diminution of the power that drives the clock, ac- 

 celerates or retards its motion, so doubtless does cleanness or foulness affect it, 

 as also heat and cold ; for all have the same effect on the jiallets and pendulum. 



The last experiments he made in 1716 and 17 18, to try what effects heat 

 and cold had on iron rods of the same length, or as near as he could to those 

 that swing seconds. He made many experiments with round rods of about a 

 quarter of an inch diameter, and with square rods, of about three quarters of an 

 inch square. The effects on both which were the same. 



