Temperatures. 287 



line, and allowed to fill. It was then drawn rapidly to the sur- 

 face and the temperature read by means^of a long-stemmed ther- 

 mometer graduated to one-fifth of a degree. The time of rais- 

 ing the bottle from the bottom of the lake was ordinarily about 

 ten seconds; and the small size of the opening prevented mix- 

 ture of the upper water with that in the bottle. The tempera- 

 ture of the water in the center of the bottle, which was meas- 

 ured by the thermometer, did not change perceptibly during the 

 time required for the thermometer to set. The water from the 

 lower part of the lake, however, was somewhat warmed by con- 

 tact with the glass and the air in the bottle. This error was 

 carefully determined by comparison with the thermophone, and 

 is about one- fifth of a degree C, when the difference between 

 surface and bottom is about 10 degrees. 



Errors much more considerable than this occur with the use 

 of the temperature bottle at the thermocline. In this region 

 the temperature may fall as many as nine degrees in a single 

 meter, and not infrequently as much as three or four degrees 

 in a quarter of a meter. It is impossible that the bottle should 

 take in all of its water from the stratum in which its mouth lies 

 as the escaping air sets up currents so that a mixture of the 

 water occurs. A difference of half a degree may therefore oc- 

 cur between the readings of the thermophone and the bottle in 

 this region. In one case the error amounted to two degrees, 

 where the bottle was opened a few inches below the upper level 

 of the cold water and took in a mixture of this water with the 

 lower part of the warm stratum above. The errors at this re- 

 gion, however, while considerable, make little difference in the 

 average results of observations, since their only effect is to 

 make the upper level of the cold water appear to be a fraction 

 of a meter lower than it really is. Since this level is subject 

 to irregular variations, under the influence of the wind, which 

 may amount to two or even more meters, the errors introduced 

 by the bottle are insignificant in the average of a week's read- 

 ings. It was intended to correct the observations of the bot- 

 tle by means of the thermophone and to introduce the correc- 

 tion in the diagrams of temperature. It was found, however, 

 that the amount of correction to be introduced in the diagrams 



