82 Chemical and Physical Notes 



moving through the air at the rate of 2 m. per second the term 

 was 17-2 seconds ; and when the velocity was i m. per second 

 it was 24-6 seconds ; while at 0*5 m. per second the term 

 was 30 seconds. Allowing that, whether moved or not, the 

 cooling of the thermometer goes on independently at the 

 still-air rate of ^5 per second, and subtracting this from the 

 reciprocals of the above numbers, we have the rates of cooling 

 due to motion of the air; at the rate of 2 m. ^, of i m. ^, 

 and of \ m. per second -^, the rate of cooling in calm air 

 being ^. The reciprocals of these fractions, or the terms, are 

 24, 40, 56 and 65, which are in the proportion 3:5:7:8. 



The importance of these figures is that they show that 

 thermometers in the open air, even when the air appears to 

 be calm, are in reality well ventilated. In a room, the 

 temperature of the dry bulb, and still more that of the wet 

 bulb, are very imperfectly given by a stationary thermometer; 

 both thermometers must be whirled in order to get anything 

 like exact observations. In the thermometer screens of a 

 meteorological station the instruments are certainly well 

 ventilated when there is a wind ; and we see that, even in 

 a calm, the ventilation may be sufficient. 



The Thermometer as a Calorimeter. The term of cooling 

 of a thermometer and the method of its determination have 

 been dwelt on at considerable length, because the information 

 regarding it to be found in manuals is usually defective. The 

 same remark applies to the thermal mass of the bulb of the 

 thermometer and the method of determining it. WJien we 

 have these two constants, namely, the term of cooling and tJie 

 thermal mass, the thermometer becomes a calorimeter, and its 

 radius of research is much increased. 



The thermal mass of the bulb of a thermometer is com- 

 pletely specified if we know (a) the weight of mercury which 

 it contains; (b) the weight of the glass which forms the 

 envelope ; (c) the specific heat of mercury ; and (d) the 

 specific heat of the particular glass used. As the mercury 

 used for thermometers must be perfectly pure, (c) is known. 

 Many different kinds of glass are used, and the information 

 regarding their specific heat is very defective. This constant 



