Birge—A Second Report of Limnological Apparatus. 535 
displacement, and the greater the difference between the height 
of the column of the mud in the pipe and the depth to which the 
pipe is driven. When, therefore, in 1915 it appeared advisable 
to ascertain the annual heat exchanges of the mud, a special ap¬ 
paratus had to be devised. 
This apparatus is an electrical resistance thermometer, which 
is mounted on the end of an iron pipe 5 cm. (2 in.) in diameter 
and about 4.7 m. (15.5 ft.) long. The case of the thermometer is 
of brass, about 9 cm. long and 5 cm. in diameter. It is conical 
at the end and provided with a steel point. The resistance coil 
is contained in the cavity of the conical tip and is sealed into 
place with water-tight cement. The whole thermometer is con¬ 
nected to the iron pipe by a piece of wood about 25 cm. long, so 
as to interpose a non-conducting material between the thermome¬ 
ter and the pipe. The entire instrument is about 5.0 m. long, and 
it can be driven into the mud to that depth. 
The resistance coil is connected with the surface by an insu¬ 
lated cable which is carried up through the pipe. The resist¬ 
ance is read by the box of a Whipple thermophone, whose scale 
is graduated into degrees centigrade. The temperature of the 
mud, therefore, is read directly in degrees. 
This pipe with the thermometer at its tip must be driven into 
the mud. From the top of the 5.0 cm. pipe extends a smaller 
pipe about 3.5 cm. in diameter and about 120 cm. long. The 
insulated cable from the resistance thermometer passes through 
this pipe, and to its upper end is attached the stirrup by which 
the whole apparatus—over 6 m. long—is suspended from this 
pipe. On this smaller pipe slides the hammer, a cylindrical iron 
weight of about 10 kg. The hammer is made of two pieces (text 
fig. 3) so that it can be put into place around the pipe after the 
thermometer is suspended in the water from its rope, and it can 
be removed again before the instrument is taken out of the wa¬ 
ter. When the two pieces of the hammer are in place on the pipe 
they are held together by two long pins (text fig. 3, A) which 
are passed through holes. These pins are longer than the diame¬ 
ter of the hammer; they are provided with a joint about 3.0 cm. 
from the end so that the tip of the pin turns down after it has 
been put in place. This arrangement prevents the pin from being 
jarred out when the hammer is worked. Two ropes are attached 
to lugs (text fig. 3, B) on opposite sides of the hammer, by which 
it can be raised and dropped so as to slide down the smaller pipe 
and strike the top of the larger one. 
