404 THE PRINCIPLES OF SCIENCE. 



the resistance of the air by operating in a vacuum. 

 Consequently this resistance has to be ascertained 

 by appropriate and tedious series of experiments, 

 which should be made if possible upon each pendulum 

 employed. 



The exact definition 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 

 different nations introduces wholly needless confusion. 

 Were all standards constructed so as to give the true 

 length at a fixed uniform temperature, for instance the 

 freezing-point, then any two standards could be compared 

 without the interference of temperature by bringing them 

 both to exactly the same fixed temperature. Unfortu- 

 nately the French metre is defined by a bar of platinum 

 at oC, while our yard is defined by a bronze bar at 62R 

 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 expansion 

 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 down, as the pressure of a column of mercury 

 is removed 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 

 compared with the bulb. For these minute causes of 

 error we may have to introduce troublesome corrections, 

 unless we adopt the simple mode of using the thermometer 

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



