NATURAL PHILOSOPHY. 191 



one, and, if not attended to, give rise to error, I think it will prove a valu- 

 able addition to a standard barometer, though never a ;.-ub.-titute for one. 

 In hopes of getting rid of the air, and of lessening the coricctioii required 

 for the force of vapor, I tried several fluids in place of pure -water. 

 Among these was oil of turpentine ; this caused a rapid evolution of gas 

 and blackening of the mercury, and depressing the column several inches in 

 a few minutes. A saturated solution of muriate of soda seemed at first 

 more successful; but in a short time the column became depressed ; and 

 this depression continuing to increase at a regular rate, the tube was 

 emptied, when it was found that the salt, having crystallized between the 

 mercury and the glass, had so allowed the air to enter. A solution of 

 muriate of lime, not being crystnllizable, was next tried ; and this seems 

 to stand best, as yet having sunk in two and a half years only .03 of an 

 inch, the greater part of this depression having occurred in the first few 

 months, giving rise to the surmise that the air which has caused it was left 

 in at the time of filling, and has not crept in since. The addition of the 

 salt to the water, besides removing to a great extent the air, has the effect 

 of diminishing the correction required for the force of vapor. The last- 

 named solution has its boiling point at 234 F., and, as has been shown by 

 experiments, the tension of vapor from water and watery solutions of salts 

 is the same at an equal number of degrees below their boiling points. The 

 correction to be applied is lessened to that of pure water 22 lower than 

 the observed temperature. This correction, which is to be added, and that 

 for the expansion of the fluids, which, is to be subtracted, thus nearly 

 neutralizing each other at low temperatures, I have applied, by means of a 

 movable scale, in the same way as is used in the sympiesometer. Among 

 the slighter variations shown by this barometer may be mentioned the 

 oscillations during a gale of wind ; these are quite as conspicuous in this 

 barometer as they were observed by Prof. Daniell in the water ba- 

 rometer, amounting frequently to 0.03, and once to 0.4 of an inch; they 

 vary in duration from five to seven seconds ; they begin with a short, 

 quick rise, followed by a slower and a much greater descent, and then a 

 return to the point of rest, which is much nearer the top than the bottom 

 of the oscillation. Previously to a gale of wind, the column descends 

 by jerks and with irregular rapidity ; but on one occasion, on which no 

 wind followed for two days, the column fell without the slightest jerk 

 more than half an inch ; there was, however, a heavy and long-continued 

 fall of rain. During heavy and sudden showers the column rises, and 

 falls again on the cessation of the shower ; on one occasion the rise was 

 .02 of an inch. In a room with a fire, with a door and window shut, the 

 column is lower than when the window is open ; the difference is usually 

 .005, but with a good fire, .01 of an inch. The last two causes are very 

 likely to give rise to error ; and the better the barometer, the greater will be 

 the error. 





