114 
lake. By the use of suitable devices it is possible to count 
the number of individuals of any kind contained in a 
measured volume of this material, and from such a count 
we may estimate the exact number of hundreds of Cyclops, 
or of any other form contained in every cubic yard of the 
water. Thus we may estimate the number of individuals 
of any kind in an entire lake. 
As there was not at hand the apparatus necessary for 
working with the vertical plankton-net, recourse was had 
to an ordinary muslin tow-net, about one foot in diameter 
and three feet deep. Since the quantity of material to be 
obtained by raising this net from the bottom to the surface 
was exceedingly small, serious errors were likely to be in- 
troduced into the measurements by the losses incident to 
emptying the net and getting the material into the pre- 
serving bottles. Instead of using the net as a vertical net, 
it was, therefore, towed behind the boat for a certain num- 
ber of minutes and then emptied. Theamount of material 
thus obtained was much greater than by using the net as 
a vertical net, and the losses incident to emptying were 
relatively so small that they could be neglected. The net 
was towed sometimes at the surface, while at other times 
it was weighted so that it traveled beneath the surface. 
When the net was out of sight -beneath the surface, the 
angle made by the tow-line was measured with a gonio- 
meter, and from this measurement and the length of the 
line was calculated the distance of the net beneath the 
surface. 
The net was usually towed behind a tug for ten minutes. 
Occasionally the time was twenty minutes and sometimes 
only five minutes, but all calculations have been reduced 
to a basis of ten minutes. The boat was estimated to travel 
at the rate of one mile per hour. For this nothing better 
was to be had than the statement of her captain, so that 
there may be a considerable error in this respect, but it is 
a uniform error which does not affect relative values. In 
