ON THE MODULUS OK TOl;s|n\.\I. RIGIDITY OF METAL WIRES. 



51 



railway, along which two troughs, one continuing the article under test imcl the other 

 a standard metre, cun IHJ moved. Tin- ini<-n scopes M. M (lig. I) are each fitted with 

 a rocking-plate micrometer,* by means <>!' which measuivinrnts t<> '0001 centim. can 

 be made. 



K.-nlw.iy 



Fig. 4. 



A U 



Y D 



Fig. 



D 



Fig. 6. 



The trough iu which the article to l>e measured is placed is a copper-lined box 

 about 120 centims. long by 18 centime, high and 10 ceutims. deep. It is divided into 

 two halves by a partition down the middle, stopping short at about 5 centims. from 

 each end. This serves to direct the circulation of the water with which the trough 

 is filled. The article under test lies on the side of this partition nearer to the slab on 

 which the microscopes are supported. The arrangement may be seen in plan in fig. 5. 

 The water is stirred by means of a fan P (fig. 5) worked by a motor Q (fig. 4). The 

 fan revolves in a box forming a depression in the floor of the trough at its middle. 

 It is situated on the opposite side of the central partition to that occupied by the 

 article whose expansion is to be determined, and drives the water up on that side, 

 thus causing a circulation in the direction shown by the arrows in fig. 5. 



The trough containing the standard metre is exactly similar to the one just 

 described. The metre is supported in a horizontal position on four rollers 25 centims. 

 apart. The first and second, and the third and fourth of these are connected together 

 by light levers having fulcrums at their centres, which are supported from the bottom 

 of the trough. This method allows of the metre being supported without bending 

 and also leaves it quite free to expand. 



* For description, see POYNTINO, 'Phil. Trans.,' A, vol. 182, p. 589, 1891, or "The Mean Density of 



the Earth," p. 95. 



H 2 



