328 Description of a differential Thermometer. 
fully bent in the form of a hook,* and the scale and foot 
being adapted, the instrument is finished. (B.) 
This thermometer is intended to be used in the same 
cases as that of Mr. Leslie, but I conceive it to possess some 
advantages. Itis more delicate. When a heated body, as 
the hand, is approached to one of the balls, the liquid sensi- 
bly ascends or descends, and as soon as this cause is re- 
moved, begins instantly to return to its former level. 
Whereas in the air thermometer, the impulsion to the liquid 
is not instantaneous, and it continues to move in the same 
direction a moment after the heating cause is removed. 
If the two balls were freed entirely from air, the liquid 
would always remain at the same level in each branch of 
the tube, except a trifling difference caused by capillary at- 
traction. This perfection cannot be obtained by the most 
skilful artist; there always remains behind, notwithstanding 
all care to prevent it, a small residuum of air, which is suffi- 
cient to make a difference in the height of the two columns. 
To obviate this inconvenience, before the scale is adapted, 
the liquid is all to be brought into one ball, and the instru- 
ment is then reversed and left for a considerable time in 
that position, that both balls may acquire an equal tempera- 
ture, and the small portion of air may be equally diffused 
through them. It is then to be restored to its proper posi- 
tion, and the point at which the liquid finally settles, is to be 
marked as the commencement of the scale. The same op- 
eration is to be repeated whenever the instrument has been 
deranged by transportation or other causes. 
If it were possible to employ constantly ether or spirit of 
wine of exactly the same degree of strength, itis plain from 
the laws investigated by Mr. Dalton, that the scale would 
be constantly uniform; but as this is not easily obtained it 
is arbitrary. I have hitherto used the division of the mil- 
limetre of France. 
Note.—This thermometer is made by Pixii Dumotiez, Rue du jardinet. 
Paris, and by Mr. Newman, Lyle-street, London. 
* The upper ball being slightly bent over to contain a small portion of 
liquid. 
+ The best mode of constructing the above instrument, isto bend the 
tube previous to the introduction of the ether, a considereble portion of 
which should be boiled out of the tube, in order to ensure the expulsion of 
