SECT. II.] COMPARISON OF THERMOMETERS. 285 



the fluid, marked a lower temperature. This is what almost 

 always happens, but this state cannot last, the thermometer 

 begins to approach to the temperature of the fluid ; at the same 

 time the fluid cools, so that the thermometer passes first to the 

 same temperature as the fluid, and it then indicates a tempera 

 ture very slightly different but always higher. 



300*. &quot;We see by these results that if we dip different thermo 

 meters into the same vessel filled with fluid which is cooling 

 slowly, they must all indicate very nearly the same temperature 

 at the same instant. Calling h, h , h&quot;, the velocities of cooling 

 of the thermometers in the fluid, we shall have 



Hu Hu Hu 



r IT* T~ 



as their respective errors. If two thermometers are equally 

 sensitive, that is to say if the quantities h and Ti are the same, 

 their temperatures will differ equally from those of the fluid. 

 The values - of the coefficients h, h , h&quot; are very great, so that the 

 errors of the thermometers are extremely small and often in 

 appreciable quantities. We conclude from this that if a thermo 

 meter is constructed with care and can be regarded as exact, it 

 will be easy to construct several other thermometers of equal 

 exactness. It will be sufficient to place all the thermometers 

 which we wish to graduate in a vessel filled with a fluid which 

 cools slowly, and to place in it at the same time the thermometer 

 which ought to serve as a model ; we shall only have to observe 

 all from degree to degree, or at greater intervals, and we must 

 mark the points where the mercury is found at the same time 

 in the different thermometers. These points will be at the 

 divisions required. We have applied this process to the con 

 struction of the thermometers employed in our experiments, 

 so that these instruments coincide always in similar circum 

 stances. 



This comparison of thermometers during the time of cooling 

 not only establishes a perfect coincidence among them, and renders 

 them all similar to a single model ; but from it we derive also the 

 means of exactly dividing the tube of the principal thermometer, 

 by which all the others ought to be regulated. In this way we 



