1 875.] Mechanical Action of Light . 243 
the bar in the opposite direction. I now remove the water, 
and the bar revolves rapidly as at first. 
From suspending the pith on a silk fibre to balancing it 
on a point the transition is slight ; the interfering aCtion of 
torsion is thereby removed, and the instrument rotates 
continuously under the influence of radiation. Many of these 
little pieces of apparatus, to which I have given the name of 
radiometers, are on the table, revolving with more or less 
speed. The diagram (Fig. 8) shows their construction, which 
is very simple. They are formed of four arms of very fine 
glass, supported in the centre by a needle-point, and having 
FIG. 8. 
at the extremities thin discs of pith lampblacked on one side, 
the black surfaces all facing the same way. The needle 
stands in a glass cup, and the arms and discs are delicately 
balanced so as to revolve with the slightest impetus. 
Here are some rotating by the light of a candle. This 
one is now rather an historical instrument, being the first 
one in which I saw rotation. It goes very slowly in com- 
parison with the others, but it is not bad for the first instru- 
ment of the sort that was ever made. 
I will now, by means of a vertical lantern, throw on the 
screen the projection of one of these instruments, so as to 
show the movement rather better than you could see it on 
2 c 2 
