Chemical and Physical Notes 



79 



the "half-fall" in the case of a thermometer, first with its bulb 

 plain, and secondly with its bulb silvered. 



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 propor- 

 tion. 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 diffe- 

 rence between it and that of the thermometer is small, than 

 when it is great. 



TABLE XI. 



The Use of the Thermometer for Measuring the Velocity of 

 Weak Cttrrents 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 



