Nichols.] 
58 
[October 6, 
in the bottle used for the collection of the water. The bottle was 
sunk to the required point and the stopper withdrawn; after the 
bottle was filled it was allowed to remain until there was certainty 
that the mass of the bottle and the thermometer had acquired the 
actual temperature of the water. The thermometers used were 
graduated to degrees, and the tenths were estimated; as the 
thermometers were compared with standards and due correction 
was made, the error is that due to reading and is probably not 
much greater than 0°.2 C. in any case, and is generally less than 
that. Most of the observations in Mystic Pond were made with 
a Negretti and Zambra “ Standard deep-sea thermometer,” which 
was graduated to fifths of a degree centigrade, and easily read to 
tenths. The corrections for this thermometer had been very care- 
fully determined by Mr. S. W. Holman of the Physical Depart- 
ment of the Mass. Institute of Technology. I found that in 
depths up to seventy-five feet, observations made by the two 
methods, with proper care, agreed perfectly, although the deep- 
sea thermometer is very much more convenient and requires much 
less time. 
For aid in the case of Fresh Pond, I am indebted to my former 
assistants, Mr. Thomas F. Stimpson, S. B., now chemist of the 
Silver Spring Bleaching and Dyeing Company, Providence, R. I., 
and Mr. John E. Hardman, S. B., now metallurgist to the U. S. 
Geological Survey, Rocky Mountain Division. In the matter of 
Mystic Pond, I was assisted by Mr. W. W. Macfarlane, S. B., now 
chemist to the Canada Sugar Refining Company, Montreal. For 
facilities at Fresh Pond my thanks are due to Mr., E. C. Brooks, 
Engineer at the Pumping Station, and at Mystic Pond to Mr. J. 
F. Symmes and to Mr. Dexter Brackett, of the City Engineer’s 
Office. 
Discussion of the Observations. 
The general results obtained from the two ponds are quite sim- 
ilar, although the ponds are differently circumstanced in some 
respects, especially in regard to the relative amount of water 
received from surface flow and through the ground. During the 
warmer months, from April to November, a considerable differ- 
