78 Chemical and Physical Notes 



units of time that have been used in expressing the time 

 interval d9. 



For the above case we have for the true term of cooling : 



R = 434 95 x 10 = 64-917 seconds. 

 0-0669 



In the application of this rule we may make the interval 

 anything we please. We may, therefore, choose it so that 

 the loss of heat during it is expressed by a simple fraction, 

 such as one-half. The convenience of this method was first 

 pointed out by Leslie, and it affords by far the best practical 

 method. It is sufficient, for instance, to heat the thermometer 

 to 10 or 11 above the temperature of the atmosphere, take 

 time when it is exactly 8 warmer than the air, and again 

 when its temperature has fallen to 4 above that of the air. 

 The excess heat present at the beginning of the interval is i, 

 and that at the end of the interval is |. Then we have : 



dl= log i log \ = 0-30103 ; 



and the term of cooling is 



0-30103 



R _ 0-434295^. 



or, very approximately, 



Hence the rule to find the term of cooling when the 

 time in which one-half of the heat excess is lost is : Multiply 

 that interval dQ by 101 and divide it by 70. The quotient is 

 the term of cooling. 



It is evident that there is no difficulty in making this 

 observation in a shop, and the time in which the instrument 

 loses half its heat is sufficient without further computation to 

 give a good idea of what has been called its thermal nimbleness. 

 The want of a term commonly used to express this important 

 property shows how much the property itself has been 

 neglected. 



Table XI gives an example of the use of the method of 



