of Edinburgh, Session 1879-80. 
537 
3. On a Differential Thermoscope founded on Change of Vis- 
cosity of Water with change of Temperature. By Sir 
Wm. Thomson. 
Water flows from a little cistern or reservoir E through a wide 
vertical tube S, about two metres long, thence through a horizontal 
capillary tube C', 50 or 100 centimetres long; thence through a 
wide horizontal metal tube T, 20 or 30 centimetres long ; thence 
through a second horizontal capillary C ; and lastly, out by a little 
constant-level overflow cup L. A vertical glass manometric tube M, 
a metre and a half long, standing up above the end of T next C 
to measure the pressure in T by a water column ; and a means of 
giving any uniform temperature to the outsides of S and C', and 
any other uniform temperature to the outsides of T and C'; com- 
plete the instrument. 
Denote the heights of the levels of the water in E and M, 
above L, by li and \h -x. If C and C' are equal and similar, or 
otherwise so proportioned as to be equal in their resistances to the 
flow of the water at equal temperatures through them, we find from 
the formula by which Poiseulle expressed the results of his experi- 
ments on the flow of water through capillary tubes — 
, *03368 . i (t - t') + *000221 . \ (f - t ' 2 ) 
* “ ? ‘ 1 + -03368 . J (t + 1') + -000221 . J (> + t' 2 ) ’ 
where t' and t denote the temperatures of the water as it flows 
through C' and C. By the arrangements described it is secured 
that t is very nearly the same as the temperature of the outsides 
of B and C. Thus, if h = 200 cms., £' = 0°, and £=1°, we have 
x = 3 *3. Thus the sensibility is 33 mms. per 1° C. ; and 1/30 
of a degree would therefore be very perceptible. 
Even with its high sensibility this instrument may not be 
frequently found convenient for thermal researches, and its chief 
use may be for illustration of Poiseulle’s important discovery. 
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