140 REPORT OP THE 



The glass plate covering the cistern is pierced between the 

 centre occupied by the barometer tube and its rim, with a hole 

 just sufficient to allow the stem of a small thermometer to pass 

 through it, the bulb of which is immersed in the mercury of 

 the cistern, for indicating its temperature. The cistern has a 

 number of small holes made through it, on a line with the 

 under surface of the plate for establishing a free communica- 

 tion with the external air ; which otherwise would be pre^ 

 vented by the mahogany rim that confines the plate in its 

 seat. 



The end of the barometer immersed in the mercury, is fur- 

 nished with a thin platinum cylinder of 0.375 inch breadth, 

 constituting Professor DanielFs platinum guard. 



The frame for supporting the tube, &c. consists of a 

 polished mahogany slab, four inches broad and thirty-eight 

 inches long, at the bottom of which is fixed a horizontal 

 bracket which supports the cistern. Another bracket is placed 

 at a height corresponding with the lower part of the brass 

 tube attached to the barometer tube, against which it is 

 clamped. The tube thus standing out from the board three 

 inches, admits of inspection throughout its whole surface 

 without removal. 



L- Fastened to the frame, is a brass plate, on which are engra- 

 ven the correction for capillary attraction ; a table for reduc- 

 tion to the standard temperature S^*^; and a correction for 

 error in micrometer screw. This error of the screw, from 

 uncontrollable circumstances, I could not avoid at the time of 

 construction, but the correction may be rendered unnecessary 

 hereafter, by the substitution of a new screw, and re-gradua- 

 tion of tlie scale; the error is attended, however, with no 

 other evil, than the trouble of applying the correction. 



The whole instrument is covered by a glass case to protect 

 it, when not in use, from all needless fingering. The mer- 

 cury used was prepared by distillation; ten pounds of mercury 

 of commerce, of specific gravity 13.619, were put into an 

 earthen retort, together with some copper and iron wire, as 

 recommended by Mr. Faraday, and 8.5 lbs. were distilled 

 over, the specific gravity of which was 13.625. This pro- 

 duct was redistilled in the same manner, and 7.5 lbs. driven 



