570 
MR. W. C. D. WHETHAM ON THE ALLEGED SLIPPING 
Time. 
Unsilvered .—Pressure 23'005 mm. ; temperature 15°‘64 ... 14 43'2 
,, Corrected for temperature and pressure . . . . 14 517 
Silvered .—Observed time. 14 49"5 
A decrease of 0'25 per cent. 
The change in radius was inappreciable. 
Thus the result of four series of experiments at these large differences of pressure is 
that in three cases the time for the silvered tube comes out slightly greater [by 0'014, 
0'44, and PO per cent.], and in one case slightly less [by 0’25 per cent.]. 
This agreement may be considered a quite satisfactory proof of identity. 
In support of his view that there is a finite slipping coefficient Helmholtz refers 
to some experiments of Girard,* who examined the time of flow of water through 
copper tubes, found that the motion was linear within limits which agree fairly well 
with those given by Reynolds’ formula, but got times of flow much less than those 
observed by Poiseuille for glass tubes. Thus with a tube whose diameter was 
1‘83 mm. and length 1790 mm., a quarter of a litre of water flowed through in 
624'5 secs, under a pressure of 100 mm. of water and at a temperature of 0°’5, while 
Poiseuille’s formula gives 2949 secs. 
As I could detect no error in Girard’s account of his experiments I determined to 
repeat them. Messrs. Elliott, of Sellyoak, Binningham, most kindly undertook to 
manufacture some solid drawn copper tubes of the dimensions required, and they were 
entirely succe.ssful. The apparatus employed was essentially the same as that used 
by Girard, and was of the simplest possible nature. Hie results depended on the 
value obtained for the diameters of the tubes, as determined by weighing when empty 
and when full of water, and as no allowance could be made for irregidarities or non- 
uniformities in the bore, calibration being impossible, it was useless to determine the 
time of flow or the temperature to any great degree of accuracy. 
A glass jar was arranged in the manner shown in the figure, and the difference of 
level between the surface of the water in A and the orifice B determined by a 
kathetometer to a tenth of a millimetre. Below B was placed a 100 c.c. flask, and 
the time taken to fill this was observed with a stop watch to an accuracy of about 
1 sec. in 400 or GoO. 
The temperature was observed in the jar A and also in B, and the two seldom 
differed by more than a tenth of a degree. In Girard’s investigations the level of 
the water in A was allowed to fall during each experiment, and its mean value 
assumed to represent the effective driving pressure throughout. In order to deter¬ 
mine whether such an arrangement vvas allowable for these small pressures, where the 
change was a large fraction of the whole, a comparison was made with a series of 
* ‘ Memoires de ITnslifcut,’ 1813-1815. 
