Viscosity of Prtch-like Substances. 349 
Experiments with Pitch. 
The greatest difficulty experienced here was the finding of a 
convenient method of preparing suitable cylinders. After 
trying various methods, the most successful was the simple 
one of rolling the pitch between boards while still soft and 
warm after recent heating. This was done on a square 
wooden board by means of a narrower board with end guides 
—hanging down over the side of the square board—which 
enabled it to be drawn parallel to itself backwards and 
forwards over the square board. Two thin strips of desired 
thickness acted as distance-pieces to regulate the final dia- 
meter of the cylinder. With a little practice very perfect 
cylinders can be rolled in this way. The next stage was to 
put squared ends to the rods. This was done by softening 
the ce and thickening and squaring the ends as shown 
in fig. 2. The ends were slightly tapered to fill the sockets 
Fig. 2. 

ee | 
on the apparatus which were also tapered. It is desirable to 
have the thickness cf the squared ends larger than that of the 
cylinder, in order that as little as possible of the observed 
flow may take place at the squared ends: otherwise the rate 
Fig. 3. Torsional Force removed. 
200 

100 
75 
50 


e5 
LE OF Twist (begrees) 

$00 ~=—-: 1000 1100 1200 
y Nae 
ANG. 
9 600 
DME f in Bis 
i bad ec of Pitch. March 16th, 1908. 
of rotation would have to be observed between two marked 
points on the bar. 
The results obtained with a specimen of pitch sold under 
Phil. Mag. 8. 6. Vol. 7. No. 40. April 1904. 2B 
