CHEMICAL AND PHYSICAL NOTES. 131 
It will be seen that silvering the bulb has in this case increased 
the term of cooling in the proportion of about 3 to 4, and the rate of 
cooling is diminished in the inverse proportion. Also for initial 
excesses of temperature between 16° and 6° the terms of cooling are 
very concordant. In both cases they increase when the temperature 
excess falls to 4°. It is to be observed that not only is the effect of 
convection less powerful at low temperatures, but any slight change 
in the temperature of the air makes itself more felt when the differ- 
ence between it and that of the thermometer is small, than when it 
is great. 
Taste XIII. 
Bulb Plain. Bulb Silvered. 
colnt : Excess of oe : 
Termet [EARS ENS | pan [TY spon, | MEE RUS| temo 
secs, secs. secs. . secs. secs. secs. 
157 109 0 16 0 137 198 
156 108 20 14 23°5 143°5 207 
157 109 45 12 54 144 208 
157 109 75 10 92 144 208 
141 98 109 8 137 146 211 
128 7 167 
162 112 154 6 198 145 209 
184 5 236 
180 125 207 4 283 150 216 
266 3 343 
332 2 433 
The Use of the Thermometer for Measuring the Velocity of Weak 
Currents of Air.—The difference between motionless air in a room 
and calm air outside is best shown and is accurately measured by the 
difference between the terms of cooling of the same thermometer as 
determined in the one medium and then in the other. This difference 
is due to the fact that calm air outside is not motionless, while in a 
room of constant temperature it is practically so. It is evident that 
if the difference is caused by the motion of the air, then that differ- 
ence must also be a measure of the motion. As has been pointed out 
above, this was perceived by Leslie, and he gives formule * for cal- 
culating the velocity of the wind from the reduction of the term of 
cooling of a tin vessel holding about half a litre of water. 
* Loe, cit, p. 283. 
