Birge—A Second Report of Limnological Apparatus. 537 
In taking the temperature of the mud, the depth of the water 
is first exactly determined by means of a wire sounding line, and 
the depth is marked on the insulated cable connected with the 
thermometer. Since this cable is not under any strain, it is easy 
to know the exact depth to which the thermometer is lowered, 
and it is easy to he certain that the thermometer has been low¬ 
ered so that its point just reaches the mud before beginning the 
readings of temperatures. Readings of the mud are usually taken 
at half-meter intervals, though other distances can readily be 
used. The change of temperature is very considerable, often as 
much as 2.0° or even nearly 3.0° in a half meter and since the 
Fig. 3. Hammer for mud thermometer. A, Pin for holding the two 
parts of the hammer together. B, Lug for attaching ropes by which 
the hammer is worked. The hammer itself is about 9 cm. long and about 
6.5 cm. in diameter. 
