504 MR BRYSON'S DESCRIPTION OF A 



entirely any chance of error when shifting the cylinder, as it will not reach the 

 wheel unless the aperture at the bottom be coincident. I is a small puppet car- 

 rying a lever, which is raised every hour by the transit of each pin of the wheel C ; 

 the axis of this lever is marked H, and the end where the pins act G. The action 

 of this lever will be better understood by reference to Figs. 2 and 3, where KK re- 

 presents a bent tube filled with mercury, forming the common syphon barometer. 

 Upon the surface of the mercury floats a spherical ball of ivory I/ attached to a flat 

 steel rod, which passes easily through a nozzle on the open end of the barometer, 

 marked P ; it is also passed through a slit in the end of the lever, which is better 

 shewn at P, fig. 3, which represents a section of the instrument. Here the float- 

 rod M is bent at right angles, and in form of a knife-edge, so as to mark the cylin- 

 der D ; K and K' are the sectional parts of the barometer tube ; H is the axis or 

 pivot of the lever ; N is the embracing arm which clasps the float-rod ; G is the 

 other arm of the lever, which is acted on by the transit of the pins. From this 

 short description, it will easily be perceived that when the wheel D revolves in 

 the direction of the arrows, each pin, as it passes the bent point of the lever at G, 

 will cause the float-rod M to be pressed against the cylinder, which removes a 

 small line of the white pigment covering the cylinder, and thus indicates the 

 height at which the float stands in the open end of the barometer. When the pin 

 has passed the arm of the lever G, it is forced into its former position by a spring 

 which gives a jerk to the float, and removes for the next observation any adhe- 

 sion of the mercury to the tube which may have been caused by moisture or 

 otherwise. The operation of marking occupies about eight minutes, during which 

 any change in the height of the mercurial column does not affect the float until it 

 is released from the embrace of the lever by the passing of the pin, when the float 

 is again free to rise or fall with every change of the atmospheric pressure, without 

 any restraint or friction, until the coming pin again brings it in contact with the 

 cylinder. 



It is convenient to have seven cylinders, each marked with a day of the week ; 

 they are quite detached, may be removed, covered, and replaced, by any person 

 totally unacquainted with the management of instruments. After the cylinders 

 are read, they require merely to be streaked with chalk and water well levigated 

 and applied by a camel's hair brush ; and as the cylinders are japanned black, 

 the slight mark made by the point of the float is very easily perceived, as it is a 

 faint black mark on a white ground. 



Fig. 4. is a representation of the reader for ascertaining the value of each 

 hour's mark on the cylinder; D is the cylinder, with the various registered 

 observations marked upon it ; a moveable pivot a presses the cylinder against 

 an opposite pivot ft, which is mounted with two milled nuts, binding it fast to 

 the upright through which it passes ; these pivots allow the cylinder to revolve 

 without any end-shake, which would vitiate the readings ; d is the scale where 



