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 sufficiently 

 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 extremely 

 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 tem- 

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



