17^ PHILOSOPHICAL TRANSACTIONS. [anNO 1/04. 



play, while they were encamped there. I struck several blows with a hammer 

 on one of them, which indeed made holes in it, but without breaking it; 

 whereas, had it been stone, it could not have withstood so many blows. At 

 last, having cut off some small bits, and made them glowing hot in the fire, 

 I observed that it was black as bone is, when it has not lain long in the fire, 

 and white when it lay longer. In this last viewing of it, I was still the more 

 confirmed that it was bone, as I could then discover the pores of a bone in it; 

 yet in my opinion it could not be made of ox- bone, as the parts were smaller. 



Experiments on the Motion of Pendulums in Facuo. By the Rev. Mr, 

 fV. Derham, F,R.S. N° 294, p. 1785. 



At my request, Mr. Hauksbee made an experiment to discover the difl^erence 

 between the vibrations of pendulums in vacuo, and in air. He provided a 

 proper receiver, and all other things necessary, such as, an eight day clock, 

 vibrating seconds, and a half seconds movement of mine. The issue of the 

 experiment was, that my pendulum vibrated two-tenths of an inch on each side 

 farther in vacuo, than it did in the free air; and went 7 seconds slower in 20 

 minutes, than the other movement. But in the open air, my pendulum in 20 

 minutes, went only 34- seconds slower than the other pendulum. 



This experiment I was desirous to try over again myself, which I did with 

 the following instruments; viz. an air-pump of Mr. Hauksbee's; the next 

 instrument was the aforesaid small movement, with a pendulum of about 10 

 inches long, that vibrates half-seconds, and is driven by the power of a spring: 

 this instrument was commodious, not only for being easily fitted with a 

 ^ceiver, but also for vibrating half-seconds very nicely, and also because its 

 vibrations are all of equal extent. The last instrument was a very well regu- 

 lated month-piece, vibrating seconds all the year, with tolerable exactness. 



Being thus provided, the result of many repeated experiments was, that in 

 vacuo, as before, the vibrations were always larger, than in the unexhausted 

 receiver. At the first, when my little movement was newly cleaned, the vibra- 

 tions were above -pV of an inch larger than in the free air. But afterwards (I 

 suppose from some of the fouled oil of the pump spirtled on the wheels, in 

 letting in the air, by which the force of the spring on the pallets was weakened) 

 as the vibrations in the unexhausted receiver were a little contracted, so in the 

 exhausted receiver they were more contracted, and only about a quarter of an 

 inch larger than in the free air. The alteration in time, which this difference 

 of the vibrations produced, was constant, only about 2 seconds in an hour 

 slower in the exhausted, than in the unexhausted receiver. For if in 4, 5 or 



