A PENDULUM FOR THE REDUCTION TO A VACUUM. 
409 
weighed about 1 1 grains * : its specific gravity I found to be 7'6 66. I was un- 
willing to use a finer wire (except with the ivory sphere), for fear of accidents, 
the issue of which could not be easily remedied in the vacuum apparatus. In 
each of these experiments the wire was attached, at its upper end, to 
the shank (1’55 inch long) of the knife edge, on which the vibrations 
were made ; in the manner described by MM. Borda and Biot : and 
the adjustment of this knife edge apparatus to a state of synchronism 
with the pendulum was always attended to. The specific gravity of 
the leaden sphere including the brass screw I found to be 11*250 ; and 
they weighed 4648 grains : of the brass sphere and screw, 7‘66 0 ; and 
they weighed 3217 grains : and of the ivory sphere and brass screw, 
1*864; and they weighed 776§ grains t ; but in all the cases where 
the pendulum has consisted of more than one metal (or even of two 
pieces of the same kind of metal, but of two different specific gravities,) 
the vibrating specific gravity of the mass has been deduced from the 
formula (2). The wire, by which the ivory sphere was suspended, was the 
finest silver wire that would sustain it with safety ; and weighed little more 
than half a grain. As these three spheres are not of precisely the same dia- 
meter, I shall designate them as the 1^ inch spheres. 
No. 5, 6, 7 are spheres of lead (No. 5), brass (No. 6), and ivory (No. 7), all 
ordered to be made of the same diameter, viz. 2*06 inches ; which was intended 
to be, and is nearly, the same size as the spheres used by M. Bessel These 
spheres were tapped in the manner already described, for the purpose of in- 
serting the screw above mentioned : and the same knife edge and iron wire, as 
those above described, were used in all the experiments. The specific gravity 
of the leaden sphere and the brass screw I found to be 11*281 ; and they 
* As a new piece of wire was occasionally found necessary, I have given what I consider the 
average weight. 
f In obtaining the specific gravities of the different substances, alluded to in this paper, I would 
observe here, once for all, that I used (not distilled water, but) river water that had been filtered 
and boiled. The values deduced are the results of two, and sometimes three, different weighings on 
different days ; and are sufficiently accurate for the comparisons intended. They are all reduced to 
the freezing point of water, and to 29'9218 inches of barometric pressure. 
1 This is the exact size of the engraving of the sphere in M. Bessel’s work ; where it is stated to 
be the true size : but on subsequently examining the detail of the experiments I found that the correct 
size is 2 - 14 inches. The plate had probably shrunk in its dimensions, since it had been printed. 
