Birge—A Second Report of Limnological Apparatus. 545 
of about 90 divisions of the scale if the shunt coil is in the cir¬ 
cuit. 
The mirror and coil are contained in the tube which projects 
from the top of the galvanometer at the rear. It can be drawn 
out and the coil locked into place for transportation. 
Method of Using the Instrument. In all of the earlier work 
with the machine the galvanometer was taken on the boat and 
operated there. Two persons are required to use it, one to operate 
the receiver and the other to read the galvanometer. This is little 
affected by the ordinary motion of the boat. It is not seriously 
disturbed by motion in the line of its main axis, but responds 
quite readily to a motion at right angles to this. It can be used 
even in a moderate breeze by bringing the boat to the windward 
side of the lake and anchoring it head to the wind, since the short 
pitching motion under these conditions does not disturb it. But 
such side motion as is caused for instance by the swell from a 
launch, even coming from a distance of a mile, sets the mirror 
swinging in a way that quite stops reading until it becomes quiet 
again. In the summer, therefore, it was often almost impossible 
to get readings on Lake Mendota, since there are at all times 
launches in motion somewhere on the lake and their swells are 
propagated over the entire surface with enough energy to disturb 
the galvanometer. It was found better, therefore, to do most of 
the work with the aid of an insulated cable, 100 m. long, and thus 
keep the galvanometer on shore. The amplitude of the swing of 
the galvanometer is practically the same whether it is connected 
with the receiver directly or through the longer cable. The ease 
of operating is much greater and the readings are more accurate 
when the galvanometer is on a stable foundation rather than in 
the boat. There is no reason why a longer cable than 100 m. 
should not be used, but in most of our small lakes 100 m. is quite 
long enough to get outside the weeds and to give sufficient depth 
of water. The instrument has been operated to depths of 10 m., 
though commonly not below 5 m. In most of our lakes the de¬ 
flection at 5 m. depth is very small—less than one per cent, of the 
deflection in air. Ordinarily it is quite sufficient to extend read¬ 
ings to the depth of 3 m. In more transparent lakes greater depths 
must be reached. 
The person operating the receiver lowers it to the proper 
depth, fastens the rope, and takes the opening cord in one hand 
35—S. A. L. 
