ME. W. CROOKES ON EEPIJLSION RESULTING FEOM RADIATION. 
333 
ments of the index ray when it should have pointed to zero gave one the impression that 
my house rested on an india-rubber cushion, so sensitively did it shift its level in obe- 
dience to a passing vehicle ; and yet it is very well built, and the part where my work 
is mostly done was erected by myself some years ago, and was made of extra strength 
for the purpose of physical research. An incredibly small angular movement of the 
base of the instrument is, however, sufficient to cause the luminous index to move. In 
a paper by Professor O. N. Rood*, “ On the application of the Horizontal Pendulum to 
the measurement of minute changes in the dimensions of Solid Bodies,” the author 
illustrates his method of determining the change of volume of bodies. The levelling- 
screw of his instrument, corresponding to screw yin my apparatus (fig. 2), rests on the 
body the change in whose dimensions is the subject of study (such as a bar of iron 
about to undergo magnetization). Professor Rood gives experiments which show that 
an increase of thickness under the screw equal to the 36 ^ 000 of an inch is an appreciable 
quantity ! 
135. The following apparatus (fig. 3) is much simpler than the horizontal pendulum, 
Eig. 3. 
k 
and is free from the objections noted above ; whilst its available sensitiveness is almost 
* Read before the National Academy of Sciences, November 4th, 1874, and published in ‘ Silliman’s Journal ’ 
for June 1875. 
3 A 
MDCCCLXXVI. 
