312 
PROFESSOR HELE-SHAW AND MR. ALFRED HAY 
found to be in agreement with the theoretical law, we did not pursue the subject of 
viscosity measurements any further. 
(c) A description of the general method of providing for the how of colour bands 
in a thin fi l m lias already been given in a paper read before the Institution of Naval 
Architects.* The apparatus, however, for doing this has been considerably improved 
and simplified, and shows in figs. 4, 5, and 6, the combination of glass plates 
which has been devised in connection with the present paper. It will be seen 
that instead of having three parallel plates of glass, only two are used, in one 
of which—made very thick—two wells are sunk, used for the clear and coloured 
liquids respectively. The figures show the construction of the apparatus, and it may 
be remarked that with this contrivance it is very easy to use a border of paper, cut 
from a template, and to employ the opposite plate of glass, or the cover-plate, for the 
purpose of providing the required well or depression through which the liquid flows. 
The depth of this well is made to correspond to the required magnetic permeability, 
the outline of the well having' the form of a cross-section of the body under investi- 
gation. How this well or depression was obtained must now be described, as the 
difficulties experienced in this case proved much greater than when a solid obstacle 
was required, as in the paper above mentioned. 
In order to solve the problem, it was necessary to obtain a transparent material 
that could be worked to any shape, and be cemented to the glass cover-plate of the 
slide to form the recess or well, corresponding in shape to the paramagnetic body for 
which the lines of induction were required. Originally, for simple circular forms, a 
sheet of glass such as is used for microscopical work, was cemented to the cover 
plate, but this was not satisfactory, as the glass could not readily be obtained of the 
exact thickness required. Further, the thickness of the layer of cement between the 
plates rendered accuracy very difficult to secure, and any figures other than circles 
could not easily be cut in glass. Celluloid had the advantage that it could be worked 
to any shape; but again the difficulty arose that it could not be obtained of the 
thicknesses required for different resistances, and moreover could not be cemented to 
the cover plate so as to lie perfectly fiat. Various papers and tracing cloths were 
tried, but they all more or less altered in thickness when moistened, and it was 
ultimately found that any material requiring to be cemented to the glass could not 
be used unless it was capable of being brought to a true surface afterwards, as no 
cement was found capable of holding sufficiently for the object m view. Plates of white 
metal were thought of, but the labour involved in producing them, and the difficulty 
of obtaining photographs of the stream-lines, led to the abandonment of this idea also. 
'fhe method finally adopted was to cast a layer of paraffin wax on the cover-plate 
of glass, and although this was not transparent enough to enable the stream-lines 
* “ Investigation of the Nature of Surface Resistance of Water and of Stream-line Motion under 
Certain Experimental Conditions.” ‘Trails. Inst. NaVal Architects,’ Vol. 10, 1898. 
