register their Indications during the Observer's Absence. 255 



no way consistent with the present advanced state of meteoro- 

 logical science. For it is admitted, that, at the present day, 

 a variation in the elevation of the mercurial column to the five 

 hundredth part of an inch, must be attended to by those who 

 aim at scientific accuracy. 



The principle of the method for registering the indications 

 of the barometer which Mr Blackadder was led to adopt, con- 

 sists in cutting off, at a given instant, all communication be- 

 tween the atmosphere and the mercury of the barometer, than 

 which, certainly, nothing can be more simple. If, at a given 

 instant, the communication between the air and the mercury 

 be cut off, the height of the mercurial column must remain 

 unaltered by any change in the pressure of the atmosphere, 

 until the communication is restored. 



A section of the barometer cistern, which is made of iron, 

 is represented in Plate VI. Fig. 1, where a is an orifice for the 

 introduction of the mercury, afterwards shut up by means of 

 a screw ; e, the air-duct, having a screw formed on its outer 

 surface ; f, an air-tight stop-cock, having a female screw, by 

 which it is attached to the air-duct ; g, a small orifice in the 

 side of the stop-cock, to serve as a passage for the air, and so 

 as to exclude dust; h, a lever connected with a time-piece, by 

 means of which the stop-cock was shut, and the communica- 

 tion of the air with the mercury cut off* at any given instant. 

 The cistern is about two inches in diameter — the depth of 

 mercury in the cistern, and the distance between the surface of 

 the mercury and the top of the cistern, must be as small as the 

 correct operation of the instrument will admit of. 



By combining several such instruments in one piece of me- 

 chanism, we can have the exact height of the barometer every 

 hour in the course of the day and night. Thus seven baro- 

 meters, arranged at equal distances around a hollow column 

 of wood, four inches in diameter, and about three feet in 

 height, having a projection at the base, in the form of a pe- 

 destal, would form not only an elegant but a very complete 

 and highly useful barometrical apparatus. The column being 

 hollow, not only lessens the weight, but permits the timepiece 

 and connecting mechanism to be entirely concealed within it. 



The barometers, however, may also be arranged on a flat 



