THE LAW OF RESISTANCE IN PARALLEL CHANNELS. 
977 
This should, he the differences in the values of log v c in Tables III. and IY. The 
actual difference is ’062. Also the differences in log i c should be the differences in 
P 3 or T 02:60, whereas the actual difference is T21. 
The errors correspond to a difference of about 1° C., which is a very probable error. 
It would be desirable to make experiments at higher temperature, but there were 
great difficulties about this which caused me, at all events for the time, to defer the 
attempt. 
Section IV. 
Application to Darcy’s experiments. 
39. Darcy’s experiments .—The law of resistance came out so definitely from my 
experiments that, although beyond my original intention, I felt constrained to examine 
such evidence as could be obtained of the actual experimental results obtained by 
previous experimenters. 
The lower velocities, up to the critical value, were found, as has already been shown 
(Art. 35), to agree exactly with Poiseuille’s formula. 
For velocities above the critical values the most important experiments were those 
of Darcy —approved by the Academy of Sciences and published 1845—on which the 
formula in general use has been founded. Notwithstanding that the formula as pro¬ 
pounded by Darcy himself could not by any possibility fit the results which I have 
obtained, it seemed possible that the experiments on which he had based his law might 
fit my law. A comparison was therefore undertaken. 
This was comparatively easy, as Darcy’s experimental results have been published 
in detail. 
Altogether he experimented on some 22 pipes, varying in diameter from about the 
size of my largest, (P'OOII up to 0 m '5. They were treated in several sets, according 
to the material of which they were composed—wrought iron gas pipes, lead pipes, 
varnished iron pipes, glass pipes, new cast iron and old rusty pipes. 
The method of experimenting did not differ from mine except in scale, the distance 
between Darcy’s gauge points being 50 m instead of 5 feet in my case. The great 
length between Darcy’s gauge points entailed his having joints in his pipes between 
these points, and the nature of his pipes was such as to preclude the possibility of a 
very uniform diameter. His experiments appear to have been made with extreme 
care and very faithfully recorded, but the irregularity in the diameters, which appears 
to have been as much as 10 per cent., and the further irregularity of the joints, 
preclude the possibility of the results of his experiments following very closely the law 
for uniform pipes. Another important matter to which Darcy appears to have paid 
but little attention was temperature. It is true that in many instances he has given 
the temperature, but he does not appear to have taken any account of it in his discus¬ 
sion of his results, although it varied as much as 20° C. in the cases where he has given 
