Mechanical Action of Light. 
233 
1876.] 
its rate of oscillation. If I relax the tension, by throwing 
the horizontal beam downwards, I get a more rapid oscilla- 
tion sideways. If I turn the levelling screw so as to raise 
the beam and weight, the nearer it approaches the horizontal 
position the slower the oscillation becomes, and the more 
delicate is the instrument. Here is the adtual apparatus 
that I tried to work with. The weight at the end is a piece 
of pith ; in the centre is a glass mirror, on which to throw a ray 
of light, so as to enable me to see the movements by a luminous 
index. The instrument, enclosed in glass and exhausted of air, 
was mounted on a stand with levelling screws, and with it I 
tried the action of a ray of light falling on the pith. I found 
that I could get any amount of sensitiveness that I liked ; 
but it was not only sensitive to the impact of a ray of light, 
it was immeasurably more so to a change of horizon- 
tality. It was in fadt too delicate for me to work with. 
The slightest elevation of one end of the instrument altered 
the sensitiveness, or the position of the zero point, to such 
a degree that it was impossible to try any experiments 
with it in such a place as London. A person stepping 
from one room to another altered the position of the centre 
of gravity of the house. If I walked from one side of 
my own laboratory to the other, I tilted the house over 
sufficiently to upset the equilibrium of the apparatus. 
Children playing in the street disturbed it. Prof. Rood, who 
has worked with an apparatus of this kind in America, finds 
that an elevation of its side equal to 1-36, 000, oooth part of 
an inch is sufficient to be shown on the instrument. It was 
therefore out of the question to use an instrument of this 
construction, so I tried another form (shown in Fig. 4), in 
which a fine glass beam, having discs of pith at each end, 
is suspended horizontally by a fine glass fibre, the whole 
