Mr Adie on the Sy'tnpiesomeUr, BU 
for such a purpose ; and I was therefore led to the idea of mea- 
suring the pressure of the atmosphere by its effect in compress- 
zing a column of common air. Upon constructing an instru- 
ment of this kind '^5 however, I found that the air was absorbed 
hy the fluid with which it was inclosed, and that a good and 
permanent barometer could not be made upon such a principle 
till this radical defect was removed. I therefore directed my 
attention particularly t® this object, and succeeded beyond my 
most sanguine expectation, in freeing the Air Barometer from 
this great source of inaccuracy. 
The name of Sympiesometer which I have given to this im- 
proved instrument, is derived from the Greek words a-vf&vtiZo)- to 
compress, and f*iTg®va measure, denoting the property it possesses 
of measuring the weight of the atmosphere by the compressiojj 
of a gaseous column. 
The principle of the Sympiesometer, wliich is represented in 
one of its forms, in Plate II. Fig. 2 ., consists in employ- 
ing an elastic fluid or gas, different from air, and any liquid, 
excepting quicksilver, which neither acts upon the gas which it 
confines, nor is perceptibly acted upon by the air, to the con- 
tact of which it is in some measure exposed. Hydrogen gas, 
azotic gas, or any of the gases not liable to be absorbed by the 
inclosing fluid, may be used; but I prefer hydrogen gas as 
superior to any other that I have tried. The liquid which an- 
swers best is an unctuous oil, or a mixture of unctuous and vola- 
tile oils. I consider almond oil, coloui^d with anchusa root, 
as the most eligible. 
The Sympiesometer consists of a tube of glass A B C, of about 
18 inches long, and 0.‘7 of an inch diameter inside, teiminated 
above by a bulb A, about two inches long inside, and bah* an 
inch diameter, (but this will vary, as the instrument is required 
to have a greater or lesser range); and having the lower extre- 
mity B bent upward, and expanding into an oval cistern C, 
open at top. 
* When I constructed this instrument, I was not aware that Dr Hooke had 
employed the compression of a column of air to measure the weight of the at- 
mosphere, The Sympiesometer, however, will be found to have no resemblance 
to his instrument but in. this parfacular. 
