A PENDULUM FOR THE REDUCTION TO A VACUUM. 
419 
1*8 inch in diameter, and 13*5 inches long; in the manner described by me 
in the paper just alluded to. The specific gravity of the lead I have assumed 
as equal to 1 1*300 : but on account of the cylindrical hole made in it, and the 
wooden rod inserted therein, I estimate the vibrating specific gravity of the 
mass as equal to 11*113 only. 
No. 41 is the same cylinder of lead attached to a flat rod of deal, 1 inch in 
width, and about 0*14 inch thick in the middle of its width, but bevilled to 
a thin edge. The cylindrical hole was (as in the preceding case,) completely 
filled with the rod, which was designedly constructed in that form at its lower 
end, in order to exclude the air which would otherwise remain in the cylinder 
and thus alter its specific gravity. The vibrating specific gravity of the mass 
is therefore the same as the preceding : and it was also suspended by the same 
spring. It was swung with its thin edge opposed to the line of motion. The 
weight of the leaden cylinder is 93844 grains. 
Results of the Experiments. 
Having thus given a description of the several pendulums employed in the 
following experiments, I shall now proceed to state the results obtained from 
each of them respectively : dividing them into different sets according to the 
form and construction of the pendulum. And here I would remark that the 
number annexed to each result denotes the number of the experiment, as given 
in numerical order in the Appendix to this paper ; where all the necessary 
information for obtaining the result, is given in detail : this mode of reference 
being considered preferable to an interruption of the narrative in this part of 
the paper. I would also previously observe that, in conducting these compa- 
rative experiments, I have always made them in pairs , on the same day , and 
immediately succeeding each other ; whereby any discordancy arising from an 
alteration of temperature of the room, or the rate of the clock, is in a great 
measure avoided : and, in continuing any series, the order of proceeding has 
been alternately reversed, which is an additional check against any error 
arising from a progressive (but unperceived and consequently unrecorded) 
variation in the rate of the clock, or the temperature of the room. Thus, 
when four experiments have been successively made (which is the smallest 
number employed,) I have swung the pendulum first in free air ; then, after 
