XV.] ANALYSIS OF QUANTITATIVE PHENOMENA. 349 



The exact definitiou of the standard of length is one 

 .of the most important, as it is one of the most difficult 

 questions in physical science, and the different practice of 

 ilifferent nations introduces needless confusion. Were 

 all standards constructed so as to give the true length 

 at a fixed uniform temperature, for instance the freezing- 

 point, tlien any two standards could be compared witiiout 

 tlie interierence of temperature by bringing them both 

 to exactly the same fixed temperature, tlnfortunately 

 the French metre was defined by a bar of platinum at 

 0°C, while our yard was defined by a bronze bar at 62°F. 

 It is quite impossible, then, to make a comparison of the 

 yard and metre without the introduction of a correction, 

 either for the expansion of platinum or bronze, or both. 

 Bars of metal differ too so much in their rates of ex- 

 pansion according to their molecular condition that it is 

 dangerous to infer from one bar to another. 



When we come to use instruments with great accuracy 

 there are many minute sources of error which must be 

 guarded against. If a thermometer has been graduated 

 when perpendicular, it will read somewhat differently 

 when laid iiat, as the pressure of a colunm of mercury 

 is remoNcd from the bulb. The reading may also be 

 somewhat altered if it has recently been raised to a 

 higher temperature than usual, if it be placed under a 

 vacuous receiver, or if the tube be unequally heated as 

 compaied witli the bulb. For these minute causes of 

 error we may have to introduce troublesome corrections, 

 \inless we adopt the simple precaution of using the tliermo- 

 meter in circumstances of position, &c., exactly similar to 

 those in whicli it was graduated. There is no end to 

 the number of minute corrections which may ultimately 

 be required. A large number of experiments on gases, 

 standard ^'eights and measures, «S:c., depend upon the 

 height of the barometer ; but when experiments in dif- 

 ferent parts of the world arc compared together we ouglit 

 as a further refinement to take into account the varying 

 force of gravity, which even Itetween London and Paris 

 makes a difference of "OOH inch of mercury. 



The measurement of quantities of heat is a matter of 

 great dilliculty, because there is no known substance 

 impervious to heat, and the proljlem is thcref(jre a3 



