286 Birge, — The Crustacea of the Plankton. 



most constant factors, and that it may be quite as accurately 

 determined as any other. 



TEMPERATURES. 



Figs. 1-5. 



The following account of the temperatures of the lake is not- 

 intended as a complete discussion of the subject. My tempera- 

 ture observations were made at first with the aim of securing, 

 approximate results in order to determine the biological rela- 

 tions of temperature. The methods employed until July, 1896,. 

 while accurate enough for these purposes, are not sumciently- 

 accurate for other ends. I have therefore refrained from print- 

 ing the observations of temperature, and discuss chiefly the- 

 temperature diagrams, which give the result of my observa- 

 tions by weekly or rather, quarter-monthly averages. 



A. Methods. 



Surface temperature observations were taken from the begin- 

 ning of my study, and temperatures from all depths after Octo- 

 ber 1st, 1894. A water bottle and thermometer were the instru^ 

 ments employed until July 27th, 1896, after which date a ther- 

 mophone was used. The latter instrument has proved extremelyf 

 useful and accurate. A full description of the instrument may 

 be found in Science, Vol. II. of 1895, page 639. As constructed 

 for my work, the instrument ranges from minus 5 to plus 30? 

 degrees C, each degree being graduated into fifths. There is> 

 no difficulty in reading the instrument to less than 0.1 degree- 

 C, and its readings are exceedingly accurate, agreeing exactly 

 with those of a standard thermometer with which it has been 

 constantly compared. Observations can be made very rapidly, 

 the time of a single reading varying from one to one and a half 

 minutes, according to the amount of change of temperature- 

 from the last reading. 



Tha temperature bottle contained about 1£ litres and had a 

 small neck. It was lowered to the desired depth; allowed to re- 

 main from one to three minutes for the glass to acquire the temr 

 perature of the water ; was then uncorked by a sudden jerk on the j 



