288 
Birge—The Crustacea of the Plankton. 
was so small as to make it inadvisable to insert it. In Figure 4 
the change from bottle to thermophone is made in the last week 
of July, and it will be seen that the lines come together with 
great accurac}^. 
Above the thermocline the bottle and thermophone agree 
exactly, except at the surface on calm, sunny days, when 
the reading of the thermometer is higher than that of the ther¬ 
mophone, since by means of the thermometer the temperature 
of a very thin stratum can be taken, while the thermophone 
coil is of such a shape that it reads only the average tempera¬ 
ture of a stratum some eight centimeters in thickness. 
During the period April — December, 1896, 189 sets of obser¬ 
vations were made on 135 days varying from 3 to 6 per week. 
In 1895, 196 sets of observations were made on 126 days in the 
same period. 
The temperature observations were made at all hours of the 
day; rarely by night, and must be taken as representing the 
day temperatures of the water. Little difference, however, 
would be made in the diagram if the night temperatures had 
been introduced, as has been shown by an elaborate series of 
observations made in 1897. Observations were regularly made 
by single meters by the thermophoue, and also by the bottle 
when the difference between single meters exceeded one-half 
degree C., and often when the differences were less. 
After recording the temperatures, those for meters not directly 
observed were interpolated, and the average was taken of the 
observations for each meter and each quar ter-month. 
In preparing Figs. 3 and 4 the average temperatures for each 
meter and quarter-month were platted at the proper depth, and 
in the center of the space representing the quarter-month on 
the diagram. The position of the full degrees was then platted 
on the assumption that a uniform decline of temperature is 
found within a single meter. This assumption is incorrect in 
the region of the thermocline as the zone of the most 
rapid decline of temperature is frequently less than a meter in 
thickness, but as this zone varies in thickness and shifts its 
vertical position under the influence of the wind, little error 
results from using this method of platting the average observa- 
