Birge—A Second Report of Limnological Apparatus. 541 
to the sun. The readings are taken on each side of the tempera¬ 
ture of the water in order to eliminate the effect of loss or gain 
of heat due to difference bewteen the thermometer and the sur¬ 
rounding water. At the proper point the observer notifies the 
recorder, who starts a stop-watch. Each 1/5 degree of rise of 
temperature is then called, and the recorder notes the correspond¬ 
ing number of seconds. When the mercury has risen as much 
above the temperature of the water as it was below it at starting, 
the recorder is notified and stops the watch. The important record 
is the total number of seconds between starting and stopping with 
the total rise of temperature in that time. The smaller intervals 
noted show whether the rise during the period is approximately 
uniform. The total exposure varies ordinarily from one-half min¬ 
ute near the surface to two minutes, or even three or more minutes 
at depths of 2 m. to 5 m. 
A good deal of work was done with this instrument, and it was 
regarded as decidedly successful. It was soon replaced by the 
electrical instrument to be described later; and therefore it was 
not employed so extensively as had been expected when it was 
constructed. I describe it so fully because I believe that it may 
easily find a useful place in limnology. It is less complex than 
an electrical instrument. It can be used without trouble from any 
kind of a boat or anywhere on a lake if the boat is anchored. It 
is easy to be sure that the bulb is properly exposed to the sun, 
and the instrument is reasonably rapid and accurate. It has been 
used without difficulty to depths of 5 m. in inland lakes, though 
at depths lower than 2 m. or 3 m. the rise is so slow as to make 
it inadvisable to go to such depths except in very transparent 
lakes. The ordinary small waves of an inland lake do not cause 
trouble, but the instrument can not be used in a rough sea. We 
attempted once to use it on Lake Superior when the water was 
smooth and there was only a slight swell, not very noticeable when 
the boat was moving. When, however, the boat was anchored, it 
lay parallel to the swell and rolled so that the eye could not be 
kept at the telescope. 
In such an instrument as this it is not practicable to employ 
the method of alternate warming and cooling and noting the times 
of both processes. The glass of the vacuum bulb and the water 
about it behave differently toward the light rays from the sun and 
the longer heat rays emitted by the black bulb of the thermometer. 
