AS DETERMINED BY FIVE PLATINUM-RESISTANCE THERMOMETERS. 
249 
McClellan in the ordinary routine, but during his vacation, or on Sundays, 
Mr. Wickham or Mr. Robinson took his jdace. All three are observers of skill 
and experience, and, as the results seem to show, the observations are of a remark¬ 
able degree of precision. 
These means were deduced from the observations as directly obtained without 
any modification or correction, other than those taken from the tables referred 
to above, except on one date — October 27—when, as I have mentioned, the short 
flexible lead was found to be affected by the dampness of the air in the observing 
room. 
This was indicated by a sudden change of about 0 o, 13 F. in the reading of the 
10 feet thermometer, which, under’ordinary circumstances changes so slowly and 
steadily, that its reading on any day might be predicted with certainty to within 
one-twentieth of a degree from the readings of two or three days preceding. On 
drying the lead, however, the abnormal readings disappeared by the next day, and 
the subsequent readings of this thermometer were found to lie along the same curve 
as before the discrepancy had arisen. 
As the dampness of the lead disturbed only the reading of the resistance box 
and in no way affected the thermometers themselves, we were, therefore, able to 
take an interpolated value for the reading of No. 5 as a standard of comparison, and 
the difference between this and the actually-observed readings, viz. : 0°*13, was 
added as a correction to all observations made on that day. 
This particular case illustrates very well the protection which the readings of a 
deep sunk thermometer afford against sudden changes occurring unobserved in the 
apparatus. 
The monthly means are graphically represented in fig. 2 ; the daily readings for two 
periods of two months each are exhibited in Plates 1 and 2. These two periods have 
been selected, as the first includes the minimum and the second the maximum of 
the 10 feet thermometer, and both illustrate very well the steadiness of the changes 
in the indications of this instrument, and exhibit also the manner in which both the 
amplitude of a wave is diminished, and its phase retarded in passing from one 
thermometer to the one below it. 
In fig. 3 are given the mean monthly temperature gradients beneath the 
surface, deduced from the same figures. 
The harmonic expression to represent the temperature of any particular 
thermometer throughout the year will be 
0 = a 0 + a x cos \t + <% cos 2 \t + &c. 
+ &i sm \t -f- sm 2A t -f- &c.. (c) 
or 0 — ct 0 + P : sin (\t -f- EJ + P 3 sin (2 \t + E 3 ) -f* &c. (cl) 
where t denotes the time represented as the fraction of a year, and X is equal to 27r. 
VOL. CXCV.—A. 2 K 
