342 THE PRINCIPLES OF SCIENCE. [CHAP. 



different temperatures. As mercury was free to flow from 

 one tube to the other by the connecting tube, the two 

 columns necessarily exerted equal pressures by the princi- 

 ples of hydrostatics. Hence it was only necessary to mea- 

 sure very accurately by a cathetometer the difference of 

 level of the surfaces of the two columns of mercury, to 

 learn the difference of length of columns of equal hydro- 

 static pressure, which at once gives the difference of den- 

 sity of the mercury, and the dilatation by heat. The 

 changes of dimension in the containing tubes bec*ame a 

 matter of entire indifference, and the length of a column 

 of mercury at different temperatures was measured as 

 easily as if it had formed a solid bar. The experiment was 

 carried out by Kegnault 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 1 



The presence of a large and uncertain amount of error 

 may render a method of experiment valueless. Foucault 

 devised a beautiful experiment with the pendulum for 

 demonstrating popularly the rotation of the earth, but it 

 could be of no use for measuring the rotation exactly. It 

 is impossible to make the pendulum swing in a perfect 

 plane, and the slightest lateral motion gives it an elliptic 

 path with a progressive motion of the axis of the ellipse, 

 which disguises and often entirely overpowers that due to 

 the rotation of the earth. 2 



Faraday's laborious experiments on the relation of gravity 

 and electricity were much obstructed by the fact that it is 

 impossible to move a large weight of metal without gener- 

 ating currents of electricity, either by friction or induction. 

 To distinguish the electricity, if any, directly due to the 

 action of gravity from the greater quantities indirectly pro- 

 duced was a problem of excessive difficulty. Baily in his 

 experiments on the density of the earth was aware of the 

 existence of inexplicable disturbances which have since 

 been referred with much probability to the action of 

 electricity. 3 The skill and ingenuity of the experimentalist 



1 Jamin, Cours de Physique, vol. ii. pp. 15 28. 



2 Philosophical Magazine, 1851, 4th Series, vol. ii. passim. 



3 Hearn, Philosophical Transactions, 1847, v oL cxxxvii. pp. 217 



