Correspondence. 417 
heel-pieces, d, d’, and in the other the graduated part of the meter is 
placed under the two microscopes, 9, o’. 
The divisions ff’ g, &c., on this meter serve to correct for tempera- 
ture and to reduce it to the standard temperature, which is that of 
melting ice. 
The above descriptions will suffice, I think, for repeating the compar- 
ns, and also for generalizing the, results, while we wait for the pub- 
lication of M. Silbermann’s memoir. The employment of Borda’s 
thermometer, which is a novelty in this department, is of great impor- 
lance, as it registers the slightest change of temperature. 
comparing Standard Weights in a Vacuum.—Before leaving 
this subject of weights and measures, I add a few words on M. Silber- 
mann’s process for comparing standard weights in the vacuum of an 
alf-pump, a-process deemed impracticable, until accomplished by M. 
ilbermann, and whose success shows that in the physical sciences 
there is nothing we need despair of. 
€ weight under trial is a brass kilogramme. Two bell-glasses 
(fig. 4) are taken, large enough to contain the weight, equal in volume 
. 
(0°20 litre) and in weight, and furnished above with a smail stop-cock. 
ch bell-glass is placed on a disk of ground glass, used as a move- 
able plate. 
The disk and bell-glass are united by the method of M. Poinset, su- 
perintendant of the Laboratory of M. Payen, at the Conservatory of 
tis and Measures; that is, by covering the edge of the bell-glass with 
a thin band of vulcanized caoutchouc from which the excess of sulphur 
has been removed by means of a potash lye. The band thus arranged, 
intervenes between the bell-glass and the plate, and if sufficiently thin, 
Vacuum is maintained in perfection. 
_ After placing the eae I kilogram in one of the bell-glasses, the 
kilogram under trial is placed in the other. ‘The vacuum is then made 
in both together by means of an air-pump ; hydrogen 1s introduced be- 
it is complete ; after expelling which, by continued pumping, the 
Pressure is so far reduced as ordinarily not to exceed two millimeters, 
80 that the weight of hydrogen left, representing this pressure, 1s not 
appreciable to the balance. : 
The weight to be considered is moreover the difference of the weights 
of the displaced volumes, a weight inferior to that of the hydrogen, 
Which is itself under this pressure only 
0:20 lit. x 0-0898 gram. X3mM. _ 9.90047 gr. 
760 mm. 
* hardly 5-hundredths of a milligramme. 
After’ putting hie! 8h cchandaal bell-glasses, each on one of the 
Scales of a balance and establishing an equilibrium by means of small 
Veights, the bell-glass containing the standard kilogram is removed and 
Teplaced by the kilogram under trial. The vacuum is made as before, 
and reduced precisely to the same degree of pressure, after which the 
bell-glass is placed on the scale of the balance and poised with a small 
Wei ht, which weight enters into the calculation. 
- Series, Vol. XV, No. 45.—May, 1853. 
