and elastic Fluids, and on I he Measurement of Temperature''. 445 



was kept up for ten or twelve minutes, which was sufficient for 

 the rods being put in equilibrium of temperature with the oil, if 

 we consider particularly that the agitation of the liquid renewed 

 every instant the surfaces in contact. (The proof of this was 

 besides acquired by the invariability of the indications of the in- 

 dex.) 



The temperature was afterwards raised to 200, 250, 300°, 

 and the simultaneous observation of the index and of the mer- 

 curial thermometer served for comparing the progress of this 

 last instrument with the dilatation of the metals. On taking 

 the difference of dilatation corresponding nearly to the first hun- 

 dred degrees of increment in the temperature, in order to ascer- 

 tain the value of the degree in our metallic thermometer, we 

 found that the term of 300^ of the mercurial thermometer cor- 

 responded nearly with 3 10^ of the metallic thermometer. These 

 experiments, which were repeated several times with results very 

 little different, prove, contrary to the opinion generally received, 

 that the metals follow in their dilatation a progress more rapid 

 than that of the mercurial thermometer. Thus by supposing 

 that we had regulated, according to the method adopted, the 

 thermometer in the air, the mercurial thermometer, and the me- 

 tallic thermometer, when the first shall mark 300° on its scale, 

 the second will indicate nearly 310°, and the third 320\ 



This property of the metals being once well ascertained, we 

 must naturally inquire if the glass, which we are obliged to em- 

 ! ploy in almost all the experiments, does not present the same 

 i phenomenon. We endeavoured to ascertain this by adding a 

 measuring rod of glass to the copper measuring rod of the fore- 

 going apparatus ; but a diificulty was then presented, which we 

 do not meet with in the metals ; the measuring rods ought to be 

 kept in a position absolutely invariable with respect to each 

 other, and this cannot be done without a screw. Now all the 

 world knows that it is im])ossible to fasten strongly a thick me- 

 tallic plate against the glass without breaking it, whatever care 

 be taken to prepare the surfaces which are to come in contact. 

 We ein])loyed in the first place, as an intermediate body, a sheet 

 of paper which had been previously brought to a temperature of 

 300° by taking care to co?npress it strongly between two metallic 

 plates. In spite of this precaution, the glass rod did not appear 

 fixed with sufficient solidity after the experiment, to remove every 

 suspicion of error. We then substituted for the paper very thin 

 lamina' of fine silver ; and for this time the immovcability of the 

 measuring; rod and of its juolongation appeared to us complete. 



We have made with this system of measuring rods several 

 series of experiments similar to the foregoing, and we found 



that 



