538 Wisconsin Academy of Sciences, Arts , and Letters. 
mud in contact with the thermometer is chilled or warmed by the 
brass point, it is necessary to drive slowly and to allow the in¬ 
strument to acquire the temperature of the mud as it goes down, 
so that the difference between mud and thermometer will be mini¬ 
mal when the depth is reached at which the reading is to be taken. 
Our regular method has been to drive the thermometer down to 
a point about 10 cm. short of the depth desired. We then wait 
until the thermometer has acquired the temperature of the mud 
as nearly as possible, and then slowly drive it down to the full 
depth. In this way the final rise or fall of the temperature is 
small. 
This thermometer was used during the winters from 1916-17, 
to 1919-20 through the ice. Observations were made during the 
summers of 1918, 1919, and 1920 from a launch, so that records 
were secured of the gain and loss of heat by the mud during the 
open season. For such observations days were chosen when there 
was little wind. The launch was secured by four anchors put 
out with long ropes in different directions, and there was no diffi¬ 
culty in keeping the boat in one spot or in working the instru¬ 
ment from the launch. 
The thermometers first made were used without accident—one 
of them for three years—until March, 1919. In that month both 
were lost in trying to extract them from the mud at the same sta¬ 
tion—12 m. depth of water. The mud here seems exceptionally 
sticky, and the thermometer was pulled out with difficulty. The 
hammer was used to help the process, by pulling it up so as to 
strike from below on the fastening of the stirrup to the top of the 
small pipe on which the hammer slides. In both cases the top 
of the pipe broke off. In constructing a new instrument, care has 
been taken to use extra heavy pipe so as to avoid similar acci¬ 
dents in the future. Steel pipe, if obtainable, would probably be 
even better. 
Table 1 is given in order to show the temperature of the mud as 
taken by the mud thermometer in winter, spring, and summer, 
in relatively shallow water. 
