115 
on the mercury, was fully justified in the event; for the 
barometer, after a few weeks had elapsed, stood at too low 
a level, and now, after an interval of 5 years, stands 3 inches 
below its proper height, while a plentiful crop of trans- 
parent prismatic crystals has been formed. 
In the barometer which I now 
venture to submit, I have applied a 
principle which I have found very 
useful in the construction of the 
manometer. Sealed within to the 
top of the long leg of a syphon 
barometer is a piece of thin glass 
rod, with the extremity of which 
at A the mercurial column can be 
brought into contact by means of the 
adjunct to the lower short leg. C is 
a tube of the same diameter as the 
rest of the barometer : it is connected 
with the short leg by means of a T 
tube, to which also the narrow glass 
tube D is attached by a piece of rub- 
ber tubing. By raising this small 
tube by turning the axle E, the mer- 
cury can be brought to touch the 
glass point at A. Affixed to the 
upper part of the framework of the 
instrument is a graduated wheel F 
having a groove in its periphery and 
also a small Y groove near its centre. 
This last holds a fine wire with a 
glass plummet at one end and an 
exact counterpoise at the other. To 
bring this plummet in contact with 
the mercury of the lower leg, the 
wheel is moved by means of the 
thread G. The contacts of the mer- 
curial column are observed with 
microscopes, and the value of the 
graduations of the wheel by com- 
parisons with a standard rule. 
