36 8 
DR, .J. P. JOULE ON A NEW DETERMINATION 
The thermometer used to indicate the temperature of the calorimeter was the same 
which I employed in my former experiments. Those designated A"' and D were 
calibrated with great care. I have recently compared them together at 50 different 
temperatures between 32° and 80° Fahr., the result being that if the less sensitive was 
assumed to be correct, the other, or A, nowhere appeared more than 0°'023 in error ; 
but taking the averages for each consecutive 10° this error amounted to no more 
than 0 o- 008. I was anxious to compare these instruments with an air-pressure 
thermometer, and with that view have constructed an apparatus in which the height 
of the mercurial column is measured by a plummet hung over the axis of a graduated 
wheel, a method which I find capable of extreme accuracy, and which I purpose to 
apply to the construction of a new barometer. But owing to the use of caoutchouc 
in the connexion between the receiver and the rest of the apparatus, I fear that the 
zero point was subject to a slight displacement. The figures at which, after much 
labour, I have hitherto arrived, could not therefore be accepted as any improvement 
on Kegnault’s determinations of the expansion of air by heat. 
The freezing-point of the standard D had risen from 13 '3 divisions of its scale in 
1844 to 1 5 ' 1 4 in 1877. I think it probable that the boiling-point of this thermo- 
meter, if kept constantly at this temperature, would in the course of time fall as 
much. The five careful determinations of this boiling-point referred to 30 bar. and 
60° are respectively 706, 7064, 706, 705'9, and 70645 — mean 706'09. Subtracting 
1*84, 704’25 will be the probable ultimate reading, from which if we take 1544 we 
shall have 68941 as the range between the fixed points cleared from the effects of 
imperfect elasticity of the glass. Mr. E. Hodgkinson has pointed outf that the 
C£ set ” of imperfectly elastic bodies is proportional to the square of the force applied. 
Therefore the effect of imperfect elasticity in the glass of the thermometers will be 
insensible for the small ranges used in the experiments, and the factor 3 '3 8 22 for 
reducing the indications of D to those of A may be confidently relied on. 
180 
We have therefore „ OA1 , „ =0 o, 07723 as the most probable value of one clivi- 
sion of A. In my former papers the number was taken as 0 o- 077214, which is so 
near that I shall continue to use it, trusting by long-continued observations of the 
fixed points to give it ultimately greater accuracy, and also by experiments above 
indicated to state it in terms of the absolute interval between these points. 
The elevation of the mercurial column in A caused by the atmospheric pressure is 
five divisions, but inasmuch as in the limited time of an experiment the barometer 
never altered 04 inch, error from this cause was neglected. The depression occa- 
sioned by capillarity was 0’33 of a division. 
A delicate calibrated thermometer E, each division of which indicated 0°41195, was 
first used for taking the temperature of the air ; but in consequence of a slight hitch 
* Phil. Trans., 1850, p. 64. 
t Brit. Assoc. Report, 1843, p. 23. 
