396 THE PRINCIPLES OF SCIENCE. 



the dilatation by heat. The changes of dimension in the 

 containing tubes now became a matter of entire indifference, 

 and the length of a column of mercury at different tem- 

 peratures was measured as easily as if it had formed a 

 solid bar. The experiment was carried out by Eegnault 

 with many improvements of detail, and the absolute 

 dilatation of mercury, at temperatures between o Cent, 

 and 350, was determined almost as accurately as was 

 needful h . 



The presence of a large and uncertain amount of error 

 may often render a method of experiment valueless. 

 Foucault's beautiful mode of demonstrating the rotation 

 of the earth by the motion of a pendulum was thus 

 frustrated. The slightest lateral disturbance of the 

 pendulum gave it an elliptical path with a progressive 

 motion of the axis of the ellipse, and this motion of an 

 unknown amount disguised and overpowered that due 

 to the rotation of the earth*. Faraday's laborious ex- 

 periments on the relation of gravity and electricity were 

 much obstructed, too, by the fact that it is almost im- 

 possible to move a large weight of iron or even lead 

 without generating currents of electricity, either by friction 

 or induction. To distinguish the electricity directly due 

 to the action of gravity from the greater quantities 

 indirectly produced would have been a problem of ex- 

 cessive difficulty. Baily in his experiments on the density 

 of -the earth was aware of the existence of inexplicable 

 disturbances which have since been referred to the action 

 of electricity with much probability 15 . The skill and 

 ingenuity of the experimentalist are often exhausted 

 in devising a form of apparatus in which such causes 

 of error shall be reduced to a minimum. 



h Jamin, 'Cours de Physique,' vol. ii. pp. 15-28. 



1 'Philosophical Magazine,' 1851, 4th Series, vol. ii. passim. 



k Hearn, 'Philosophical Transactions,' 1847, vol. cxxxvii. pp. 217-221. 



