Adie's Syrnpiesometer. 
201 
IV — Remarks on Adie's Syrnpiesometer. 
This instrument is now in frequent employment ; and although it can never he ren- 
dered equal in accuracy tothebarometer, stillinsome cases, and particularly at sea, it 
hasbecome a very useful substitute for the latter, and therefore any hint towards perfec- 
ting its construction should be acceptable to the inventor as well as to the public. 
The Syrnpiesometer tells the changes of the atmospheric pressure, by its action 
upon the volume of gas contained in a simple air thermometer. There are neces- 
sarily two scales to it ; the barometric scale of inches being made to slide over the 
thermometric scale, so as to furnish an adjustment for every change of temperature, 
which latter is ascertained by a very delicate mercurial thermometer enclosed in the 
same case. 
Mr. Adie’s description says, that both of these scales are graduated “ by experi- 
ment,” a fully sufficient cause for the irregularities which I have noticed in many 
of them. In one made by Dollond, the thermometric degrees alone were 25 per 
cent, too small ; in short this is just one of those instruments, the graduation of 
which by the maker should not be trusted before a careful re-examination has been 
made;foritmay be laid down as a general rule, that unless the divisions are unalter- 
able, — as inches ; — or easily limitable, — as between the fixed points of a common 
thermometer, — they are for the most part imaginary, and placed at random by the 
maker to save himself the trouble of delicate experiments, which in many cases it 
may not be iu his power to perform. 
In the case of Ihe Syrnpiesometer, it is first necessary to verify with great nicety 
the thermometric degrees ; the barometric scale may then be easily deduced by cal- 
culation as follows : 
) . All gases expand .00208 of their volume for each degree of Fahrenheit’s ther- 
mometer* : 
2. The volume of gas is constantly in the inverse ratio of the barometric pres- 
sure ; that is, 
The bulk of gas under a pressure of 31 inches being 1,0000 
The same under the pressures 30 will be “ 1,0333 
29 1,0688 
28 1,1072 
27 1,1485 
To express these differences (.0333, .0688 &c.) in terms of thermometric de- 
grees, it is merely necessary to divide them by .00208 ; thus 
i|. 31 inches to 30 = -iJH = 16°,05 | 
a a n 
.21 3 
B ^3 £ 
S 
^ CG 
Sts £ 
Eh 8 a 
3 
31 
31 
29 = 
28 = 
27 = 
,0688 
,00208 
,1072 
,00208 
,1485 
= 33,07 
= 51,54 
= 71,38 
§ S 
,00208 
Two instruments tried by myself had these divisions in the following erroneous 
proportions : 
16,6 33,4 51,1 and 69,3 
15,4 31,3 48, 65,0 
There can be no doubt, that the theoretic divisions are alone correct, and thatthere 
can be no occasion to verify them experimentally. 
It is, however, a material defect in the Syrnpiesometer, that the two scales made 
on the foregoing principles can only be mathematically correct, for one constant 
prepare, and one determinate temperature : for as the expansion from either cause 
is directly in proportion to the total bulk of the gas ; if this bulk is increased bv a 
diminution of pressure in the atmosphere, then the thermometric degrees should be 
increased at the same rate ; or if the bulk be expanded by an increase of heat then 
the inch scale should be expanded in the like proportion. 
Assuming, therefore, that the thermometer is graduated for the standard pressure 
of 30 iifches, the degrees under other pressures will bear the following ratios. 
* i. e. 00208 of their volume at 32°,-»Ed, 
