OF ARTS AND SCIENCES. 155 



General Criticism. 



All the results so far obtained, except those of Joule, seem to he of 

 the crudest description ; and even when care was apparently taken in 

 the experiment, the method seems to he defective, or the determination 

 is made to rest upon the determination of some other constant whose 

 value is not accurately known. Again, only one or two observers have 

 compared their thermometers with the air thermometer, although I 

 have shown in "Thermometry" that an error of more than one per 

 cent may be made by this method. The range of temperatures is 

 also small as a general rule and the specific heat of water is assumed 

 constant. 



Hence a new determination, avoiding these sources of error, seems 

 to be imperatively demanded. 



.{!>.) Description of Apparatus. 

 1. Preliminary Remarks. 



As we have seen in the historical portion, the only experiments of 

 a high degree of accuracy to the present time are those of Joule. 

 Looked at from a general point of view, the principal defects of 

 his method were the use of the mercurial instead of the air thermom- 

 eter, and the small rate at which the temperature of his calorimeter 

 rose. 



In devising a new method a great rise of temperature, in a short time 

 was considered to be the great point, combined, of course, with an 

 accurate measurement of the work done. For a great rise of tem- 

 perature great work must be done, which necessitates the use of a 

 steam-engine or other motive power. For the measurement of the 

 work done, there is only one principle in use at present, which is, 

 that the work transmitted by any shaft in a given time is equal to 2 it 

 times the product of the moment of the force by the number of revo- 

 lutions of the shaft in that time. 



In mechanics it is common to measure the amount of the force 

 twisting the shaft by breaking it at the given point, and attaching the 

 two ends together by some arrangement of springs whose stretching 

 gives the moment. Morrn's dynamometer is an example. Him* 

 gives a method which he seems to consider new, but which is immedi- 

 ately recognized as Huyghens's arrangement for winding clocks with- 



* Exposition dc la Theorie Mccaniquc dc la Clialeur, 3 me cd., p. 18. 



