Nichols.] 
70 
[October 6 
In January, 1880, a similar rise of temperature occurred while 
the pond was covered with ice. Thus the mean of the three 
observations made at different depths on January 5, was 2°.3, 
while on January 26 it was 3°.0. During this interval, the water 
rose 0.07 foot in spite of the usual pumping, so that a very con- 
siderable amount of water must have entered the pond during 
this time. 
If, now, we turn from Fresh Pond, which is known to receive 
much “ground-water,” to Mystic Pond, which is fed mainly by 
surface-water, we find a different condition of things. In the 
case of Mystic Pond, it would appear that the cooling continued 
to an appreciable extent after the ice had formed; thus the mean 
of the observations taken December 19, 1879, three days before 
freezing, was 3°. 5 ; the means of the observations taken through 
the ice were as follows: January 30, 1880, 3°.l ; February 6, 2°.4; 
February 17, 2°.3; February 18, 2°.6. These are the means for 
the observations at three different depths, but it will be seen (Table 
II) that the variation is mainly in the surface layer, so that we 
ought really to consider the surface-water and the deeper water 
separately. 
First, as to the surface-water. From the time of freezing, 
December 22-23, until the time of the first observation through 
the ice, January 30, the water was rising rapidly in the pond, but 
from that time until February 18, the level was nearly constant, 
and during that time the temperature of the surface-water cer- 
tainly fell, as appears very clearly in Table III, where we have 
the daily observations of the water received in the pump-well of 
the water-works. The bottom of the conduit was, at this time, 
about ten feet below the surface of the pond. It is to be noted 
that the bulk of the water enters the main pond at the end 
furthest from the gate-house through a shallow passage, conse- 
quently near the surface; being, most likely, cooler than the 
mass of water in the pond, it would tend to flow over the surface 
just under the ice. 
If now we consider the water from a depth of eighteen feet to 
