of Magnetical and Meteorological Instruments. 329 
wet edges of the external glass cylinder; lastly, we are enabled, 
by the employment of the sliding screens described below, 
make use of nearly the whole width of the paper for barometric 
changes, instead of being restricted to about one-half of it. It 
should be added that by making a shallow well in the fixed half 
of the cylinder, a large surface of wet lint may be exposed imme- 
diately under the paper, which is found to keep it damp enough 
under all circumstances, even when the slits are open. 
The barometer employed, isa syphon, “constructed with a 
column of mercury of a little more than one inch in diameter. As 
the weight of an entire column of this size would be inconven- 
ient, and as it would be difficult to obtain a tube more than 
three feet long of so large a bore, both ends of which were of the 
same internal area, two adjacent short pieces of a very nearly cyl- 
indrical tube, have been united to the extremities of a tube of small 
bore, and form the ends of the instrument which contain the sur- 
faces of the mercury,” thus shewing the variations of pressure by 
an equal change of half the amount in either tube. These 
changes are communicated to a long and slender index or lever 
by means of a float, attached to a short arm at right angles to it, 
both being centred on a light wheel of metal, carefully pivoted, 
and both being counterpoised. The float actually employed is 
the bulb of a mereury thermometer, the stem of which passes 
through a cap adapted to the open end of the syphon, and is gnid- 
ed by three small friction rollers.* A screen of card or very thin 
metal, provided with a narrow slit exactly thirty inches frem 
the centre of motion, is attached to the upper end of the long 
index, covering an opening in the copper case of the cylinder; a 
lamp being then fixed behind the screen, the pencil of light which 
falls on the paper through the slit left for the purpese, will, it is 
evident, follow every change in the position of the index, or of the 
Surface of mercury in the syphon. A lens similar to that which 
_ Is used to bring the elongated image of the slit reflected from the 
mirror of the magnet to a point, is also nsed here to bring the 
Image of a horizontal slit in a corresponding screen before this 
lamp, to a focus on the paper. It forms a bright line across its 
whole width, which is however intercepted by the barometer 
Screen on one side, and by an independent screen on the other, 
allowing to reach the paper, only the variable pencil passing 
through the former and an invariable one, rpose of 
tracing a base line through a fixed opening in the latter. 
The tube of the barometer has a vertical movement, to allow 
an adjustment of the level of the mercury at the foot of the sy- 
phon, to the same horizontal plane, so that whatever be the pres- 
* These have been generally dispensed with, at Toronto, and the end of the 
Syphon left open : unless very truly turned, and perfectly centred, they are apt to 
impede rather than assist the motion of the stem of the float. 
Srconp Serms, Vol. IX, No. 27.—May, 1850. 
