EXPERIMENTS WITH VIBRATING CILIA. 615 
or shorter than this, as may be convenient to the experimenter. 
Behind the instrument there should be a small black board at- 
tached to the base on which the frame rests, to serve as a back- 
ground against which the pointers are seen. The instrument is of 
sufficient delicacy to be moved by a weight of from one hundred to 
one hundred and twenty millegrams or of from seven to eight 
grains. All that is necessary to make an experiment is to attach 
the end of the thread coiled around the drum to the hook on the lead 
which rests on the piece of skin, which in turn rests on the mem- 
rane. The proportions of the wheels are such, that when the 
load resting on the membrane is carried through a space of 7mm. 
the index makes two whole revolutions, and, the point of the in- 
dex moves through a space of about two feet. One complete 
revolution is effected in about thirty seconds. This motion may 
be easily seen over a large lecture room. 
Fig. 110. 
Description of the diagram. a. A movable block of wood to 
which the ciliated membrane is pinned. b. Ciliated membrane. 
c. Piece of skin resting upon it. d. Plate of lead with a small 
hook to which the thread coiled around the drum is attached. e. 
and f. Pointers, one resting on the lead and the other on the 
board to which the instrument is fastened ; these are made of wire 
inserted into a base of wood or cork, and carry each a piece of 
paper or thin card; both are movable. The wheels are toothed. 
Attached to the axis of the large wheel is the drum, and to the 
projecting portion of the axis of the small wheel is the index. 
This figure is one-half the size of the instrument. The base on 
