EXPANSION OF SOLIDS WITH RISE OF TEMPERATURE. 21 



shown ; the other end pressed against a lever CL, which could rotate 

 round the axis C, supported in bearings on two other pillars, of which 

 again only the back one is shown. To the axis of the lever CL was 

 attached a telescope about 6 feet long, directed to a vertical staff 

 divided to twelfths of a French inch, and distant 600 feet from the 

 telescope. The temperature of the trough was raised by a furnace 

 underneath it, but from their size and distance from the furnace the 

 pillars were unaffected. The axis 00, therefore, remained a constant 

 distance from the fixed piece TFF. Hence, an expansion of the bar 

 moved the end L of the lever and rotated the telescope, the length CL 

 being such that an expansion of the bar through one line moved the 

 cross-wire in the eyepiece over 744 divisions of the image of the staff. 

 This must not be taken to imply that the accuracy was increased 744 

 times, for the passage of the cross-wire over one division of the scale 



FlG. 13. Diagram of Expansion Apparatus of Lavoisier and Laplace. 

 Front pillars not shown. To a crosspiece T, supported horizontally 

 across the two left-hand pillars, was attached the fixed vertical rod 

 FF ; ft were two other crosspieces to which were attached vertical 

 rods carrying rollers rr. Another crosspiece served as the axis of the 

 lever CL and the telescope. 



probably could not be estimated nearly so accurately as an increase in 

 length of one line in the bar when directly observed.* 



Modifications of the method have since been made by Miiller and 

 others, in which the telescope is replaced by a mirror on the axis 0. The 

 reflection of a fixed scale is viewed in the mirror by a fixed telescope, 

 and the motion of the image of the scale across the field of view through 

 the rotation of the mirror gives the expansion of the bar. The bar 

 abuts against small rounded projections attached to the vertical pieces 

 at each end, so that it touches the lever CL at a fixed point. The adop- 

 tion of the mirror method has two great advantages : (1) It reduces the 

 weight of the moving parts by the substitution of a light mirror for a 

 heavy telescope ; (2) it economises space, for the telescope and scale may 

 be placed at half the distance of the scale in the original experiment, 



* The details of Lavoisier and Laplace's work were not very fully published, and 

 it is not known whether they took sufficient precaution to maintain the length CL 

 of the lever invariable or not. 



