EXPERIMENTS ON SURFACE-FRICTION. 249 
but that measurements made after an interval of a few months, even when 
the pyrometer had not been heated in the mean time, sometimes differed deci- 
dedly from the results previously found. Possibly such changes may be due 
to alterations of the unsoldered connexions of the conducting-wires ; but, what- 
ever their cause, they would probably be met with in actual practice if the 
pyrometers were used during long periods of time. 
Report to the Lords Commissioners of the Admiralty on Experiments 
for the Determination of the Frictional Resistance of Water on a 
Surface, under various conditions, performed at Chelston Cross, under 
the Authority of their Lordships. By Wit4114M Frovupn, F.R.S. 
[A communication ordered by the General Committee to be printed in extenso.] 
(Puates VIII.-XII.) 
Second Report*. 
Chelston Cross, 
13 December, 1872. 
As in the Report on the subject handed in in August last, the results of the 
investigation will be presented under three principal aspects :— 
(1) The law of the variation of the resistance, in terms of the variation of 
the speed. : 
(2) The law of the variation of the resistance, in terms of the variation 
in the length of the surface. 
(3) The nature of the variation of the resistance, in terms of the variation 
in the quality of the surface. 
It will be seen, however, that, as exemplified by the results now presented, 
no less than by those presented in the former Report, the three laws are more 
or less interdependent. 
In this concluding part of the series it was sought to give completeness 
to the determination of the effect of quality, in what may be termed its 
practical extremes of smoothness and of roughness. The experiments com- 
prising the completion of the trials made with a tinfoiled surface on the one 
hand, and one coated with rough sand on the other, represent these extremes. 
The list of materials used in forming the surface includes (1) tinfoil; 
(2) hard paraffine, laid on thin and scraped perfectly smooth (this was also 
used as a substratum on which to lay the foil, the medium of adhesion being 
a thin coat of tallow) ; (3) blacklead, polished on the paraftine; (4) unbleached 
calico; (5) three varieties of sand, differing from one another in the coarse- 
ness of grain. The sands, of graduated fineness, were in turn sifted on to a 
paraffined surface, having been previously sufficiently heated to melt their 
way into it and become fixed there. 
There was, as might be expected, some difficulty in securing identity of 
quality (1) throughout the length of each individual surface, and (2) (a for- 
tiorz) in the planes of different length. Of the smooth surfaces, the scraped 
paraffine, naked, was perhaps the most uniform for all lengths; of the rough 
* For Preliminary Report vide Report of Brighton Meeting, 1872, p. 118. 
