xv.] ANALYSIS OF QUANTITATIVE PHENOMENA. 343 



are often exhausted in trying to devise a form of apparatus 

 in which such causes of error shall be reduced to a 

 minimum. 



In some rudimentary experiments we wish merely to 

 establish the existence of a quantitative effect without 

 precisely measuring its amount ; if there exist causes of 

 error of which we can neither render the amount known 

 or inappreciable, the best way is to make them all 

 negative so that the quantitative effects will be less than 

 the truth rather than greater. Grove, for instance, in 

 proving that the magnetisation or demagnetisation of a 

 piece of iron raises its temperature, took care to maintain 

 the electro-magnet by which the iron was magnetised at 

 a lower temperature than the iron, so that it would cool 

 rather than warm the iron by radiation or conduction. 1 



Eumford's celebrated experiment to prove that heat was 

 generated out of mechanical force in the boring of a 

 cannon was subject to the difficulty that heat might be 

 brought to the cannon by conduction from neighbouring 

 bodies. It was an ingenious device of Davy to produce 

 friction by a piece of clock-work resting upon a block 

 of ice in an exhausted receiver ; as the machine rose in 

 temperature above 32, it was certain that no heat was 

 received by conduction from the support. 2 In many 

 other experiments ice may be employed to prevent the 

 access of heat by conduction, and this device, first put in 

 practice by Murray, 3 is beautifully employed in Bunsen's 

 calorimeter. 



To observe the true temperature of the air, though 

 apparently so easy, is really a very difficult matter, because 

 the thermometer is sure 'to be affected either by the sun's 

 rays, the radiation from neighbouring objects, or the escape 

 of heat into space. These sources of error are too fluctu- 

 ating to allow of correction, so that the only accurate mode 

 of procedure is that devised by Dr. Joule, of surrounding 

 the thermometer with a copper cylinder ingeniously 



1 The Correlation of Physical Forces, 3rd ed. p. 159. 



2 Collected Works of Sir H. Davy, vol. ii. pp. 12 14. Elements of 

 Chemical Philosophy, p. 94. 



3 Nicholson's Journal, vol. i. p. 241 ; quoted in Treatise on Heat 

 Useful Knowledge Society, p. 24. 



