METEOROLOGICAL INSTRUMENTS. 121 



The Mountain Barometer. (Fig. 608, next page.) A 

 very important property of this valuable instrument is that 

 of determining heights, and its accuracy is inferior to no 

 other mode ; on this account it is not only very useful to 

 surveyors, but highly interesting to philosophers, scientific 

 men, <fec. It consists of a tube, &<?., as described in the 

 first article under this head, having a smaller reservoir for 

 the mercury, which is inclosed in a round case, about one 

 and a half inches in diameter, at the upper part of which 

 an opening, of about one quarter of the circumference of 

 the case, and twelve inches long, is made, the sides of 

 which are inclined towards the tube, on one of which the 

 barometer scale is attached, which ranges from twenty to 

 thirty-one inches, divided to twentieths of an inch, and hav- 

 ing a vernier sliding on the scale, subdividing these into five 

 hundredths of an inch ; to the upper edge of the vernier there 

 is a small index to be set in a line with the mercury ; on 

 the other side of the opening there is a delicate thermo- 

 meter, which may be detached if required ; that part of the 

 case to which the scales are attached is surrounded with a 

 brass tube, having an aperture cut in it corresponding to 

 the opening in the case ; there is also a narrow opening at 

 the back through the case and tube, for allowing the light to 

 be seen through, and more accurately observing the surface 

 of the mercury ; the tube may be turned round so as to 

 close these openings entirely, and is kept on by a brass cap 

 at the top, into which a ring may be screwed for suspending 

 the instrument. 



As the reservoir in this instrument is small, and any con- 

 siderable depression of the mercury would considerably alter 

 the level therein, a correction is made in the best instruments, 

 for which purpose the reservoir is accurately measured, and 

 the contents of one inch, compared with one inch of the 

 tube, ascertained, which is to be subtracted from the height 

 when the mercury falls below thirty inches, which amount 

 of correction is engraved on the brass cap. The reservoir 

 is inclosed in a brass case, having a portable screw, over 

 which the brass cap is screwed. This barometer has been 

 used by many scientific gentlemen in their observations, and 

 is the least liable to derangement of any used for mea- 

 suring heights ; is very light, and may be inverted, and 

 with care used as a walking stick in ascending heights. 



The density or weight of the atmosphere is not equal, but 



VOL. II. 11 



