On Sensible Temperature, 
[Mat? 
power of the air, as of a lower temperature than the body; the cooling power 
of the air in motion, or of the wind ; and the cooling power of evaporation 
from the moistened surface of the skin ; .all contributed to the result. The feelings 
were noted as rather oppressive. Soon after they became more bearable, at least 
while sitting still and making no exertion, being at the same time in a very light 
undress. The experiment being now repeated, gave 18° as the fall in one minute: 
the temperature of the air was 81°, of a moist surface 73°. In the evening, when 
the sensations were quite agreeable, the fall due to one minute was found 21°. In 
the first of these cases it is evident that heat was generated faster than it could be 
carried off, while in the last the balance must have been nearly adjusted. The pro- 
portion of the heat lost, in these two cases, is nearly as 2 : 3. 
The following day having occasion to try again what result I could obtain when 
the feelings were rather agreeable in a dress of calico, such as we wear in India, in 
the warm weather, I obtained 23° as the fall in one minute, hut observed that during 
the last 10 or 15 seconds, the thermometer was stationary, having, in fact, reached 
the temperature of the air. This fact indicates the necessity of shortening the time. 
The enquiry, in strictness, belongs to a mere momentary decrement ; but in practice 
it is necessary to take a sensible interval, not only to have an appreciable scale of 
time, but also of the cooling effect. By a well known theorem it is easy, if necessa- 
ry, to reduce these quantities to the momentary decrements. In the case in ques- 
tion I tried 20 seconds, and found the fall to be 11°, which may, with this thermo- 
meter, be said to represent that state of atmosphere in which a calico dress is re- 
quired, but is yet abundantly cool. The temperature of the air was 78°, o', and of a 
moist surface 69°, there was scarcely any wind. In the evening again this thermo- 
meter fell 13° in 20 seconds, but as yet the calico dress was not found too cold, nor 
even afterwards with a fall of 17° in the same time: — perhaps in this case the per- 
son was not exposed for a sufficient length of time to make the experiment satis- 
factory. 
On entering the house, the fall due to one minute was found to be 18°. This was 
the quantity which had been observed the preceding day, but the feelings were 
certainly not so agreeable as in the present case. The temperatures were, in these 
cases of the air, 80 and 72 ; of an evaporating surface, 72 and 67. Though the fall 
of the moist thermometer was then the same, yet that of the dry one would have 
been very different. This induces a suspicion that the skin does not part with its 
beat so rapidly in proportion as a moist thermometer, though more rapidly than a 
dry one ; a conclusion which, on subsequent consideration, 1 find evident, inasmuch 
as its moist surface is not exposed directly to the cooling power of the air, but 
t lough, the interposition of dry clothes. This very obvious idea did not strike ine 
at the time, and I proposed to get over the difficulty by observing both thermome- 
ters, and then combining their results in such a manner as future observation might 
su ggest. The remedy, however, is much simpler : it is only necessary to cover the 
moist bulb thermometer with a loose envelope of calico ; its indications will then 
orni a more accurate register of the feelings. An experiment tried immediately 
after wards with the dry thermometer, sufficiently shows the fallacy of the infor- 
mation given by the ordinary indications of this instrument. The temperature of 
the air, within the house, was within 1 ° of what it was outside ; yet the feelings were 
so different, that in the one case they were agreeable with a calico dress, in the other 
they felt absolutely cold, and a woollen dress became desirable. In these cases, the 
tad within the minute of the dry thermometer, raised to the temperature 98°, was 
about 8 ,m the other 16° ; showing that the heat was dissipated twice as rapidly in 
the latter as in the former case. 
The following day another experiment confirmed the preceding view. The moist 
thermometer, raised to 98o, fell 21° in a minute, yet the feelings were barely tolc- 
f e ,' , n tr T ln g the dry one, I found it had a depression of but 7°, whereas I 
many subsec l uent trials that it required 10“ at least, with this fall of the 
insert , l °c corre . s P on <} with any thing like agreeable feelings. I shall 
matters L ^ ^ informatlon of such of your readers as may be curious in such 
Fall in 1 Minute. 
Dry Thermometer. 
10 
11 
9,5 
4, 
6,4 
II 
Thermometer raised to 98®. 
Fall in 1 Minute. 
Moist Thermometer. 
21 
21 
13.5 
15.5 
27 
Remarks. 
Feelings agreeable. 
Pleasant and cool. 
Calico dress just bearable. 
Very oppressive. 
Oppressive even in undress. 
Woollen clothes bearable. 
