NORTHWESTERN LAKES OF THE UNITED STATES. 
55 
Negretti-Zambra deep-sea thermometer is sealed in a second glass tube, surrounding 
it with an air chamber so that is it not affected by the increase in pressure in deep 
water. This air chamber is partly filled with mercury, which conducts the heat 
to or from the bulb of the thermometer. 
The thermometer used in 1913 was standardized by the United States Bureau 
of Standards, and the corrections given have been applied on the deep lakes. 
Fig. 2.— Anchor release. 
Great care was taken to read the thermometer at the temperature of the surface 
water, and the correction for the expansion of the column of mercury at that tem- 
perature was applied. These thermometers were graduated to 1° F., and the 
fractions were estimated. After the first set of temperatures on Crater Lake was 
taken the standardized thermometer was checked with a standard thermometer at 
the University of Washington and was found to agree within 0.1° C., which is con- 
sidered the limit of error in reading. 
