120 G. H. KNIBBS. 
t, that at 0° being unity, is therefore simply 1+7 x. For the ait 
thermometer the coéfficients are respectively 181792, 0°175, and 
0-035116, according to the same authority. f 
Dilatation of Glass.—Poiseuille adopted! for the coéfficient of 
cubic dilatation for glass, £, Dulong and Petit’s estimate viz., 
€=0-0000258 (32) 
to which, though somewhat less than later determinations,” I have 
nevertheless adhered as being sufficiently exact for the purpose 
in hand. 
Thermometry.—The degree of rigour in the thermometri¢ 
element of the various measurements of viscosity has not beet 
explicitly indicated by each observer. Most of the measurements 4 
however are to an order of precision which rendered this of little 
moment. In the future it is to be hoped that rigorous methods 
of thermometry will be employed’ and that the determination will 
be carried to a higher degree of precision: merely verificatory 
Measurements are of little value at the present stage of the : 
investigation. : 
a 
18. Poisewille’s experiments 1846. Volumes of the effin. the 
volume discharged in a given time was measured by the content | 
of bulbs, such as that shewn in Fig. 4, in which V denotes ™ 
bulb. These were filled with mercury and weighed with a precisio® 
of about 0°5 milligramme,‘ the temperatures being recorded. The 
results were then reduced so as to express the volumes at ee 
Poiseuille assumed the density of mercury at 11°.5—the temp® 
ture of one of the experiments, vide p. 515 of his memoir—to 
13-569675. By the constants used by me this should be 13-567 
: Mém. des Sav. étrang. t. 9, p. 515 et 526, 1846. 
For the cubie dilatation of glass 0° - 100° C, Rossetti and Levy ® 
B ai 
0000262, “0000267, and the mean of Regnault, Wiillner, and Levy's 
is 0000269 : 
