1880.] 
59 
[Nichols. 
ence in temperature exists between tbe top and the bottom of the 
pond and the temperature decreases regularly from top to bottom. 
It is true that the water which enters at the bottom and sides of 
the pond is, during this time, colder than the surface water, but 
this is not the main cause of the difference. The chief cause lies 
in the fact that water is almost a non-conductor of heat; conse- 
quently the upper layers warm up much more rapidly in the 
spring although, in this climate, at a depth of seventy-five feet 
the difference between the summer and winter temperature is 
considerable. 
In shallower ponds, in storage reservoirs, and in warmer latitudes 
we should expect less difference between top and bottom. Thus 
Mr. Fteley 1 found in one of the storage basins on the Sudbury 
Kiver, from October 1 to December 20, 1879, almost identically 
the same temperature at the top and at the bottom where the 
water was twenty-five feet deep. This is true, in general, of very 
shallow ponds and of ordinary rivers, also of the shallower portions 
of even very large lakes. For example, the observations of the U. S. 
Signal Service (unpublished), made at various points along the 
shores of the Great Lakes, show almost no difference (and, at some 
stations, absolutely no difference from the beginning of the year to 
the end) between the top and bottom temperature at the same time, 
but the observations are made close to shore, generally from some 
wharf or pier, and seldom in a depth greater than fifteen feet. Alex- 
ander Agassiz found in Lake Titicaca, Peru (Lat. about 16° S., Long, 
about 70° W.), that “ the usual difference between the surface and 
the bottom, even at the greatest depth (154 fathoms) was not 
more than from three to four degrees (Fahrenheit). The lowest 
temperature of the bottom was only 51° (10°.6 C.), the general 
temperature varying from 54° to 55°; while the surface tempera- 
ture ranged from 53° to 59°, the greater part of the time 56° to 
57° Fahrenheit.” 2 These observations were made in February, 
1875, when the temperature of the air at midday was from 49° to 
97° Fahrenheit. 
1 First Annual Report of Mass. State Board of Health, Lunacy, and Charity. Sup- 
plement, pp. 121-128 and plate. 
2 Proc. Amer. Acad., xi (1876), pp. 283-292. 
