the Investigation of Young's Modulus for Wires. 



547 



disposed tapering springs of brass clamped at their thick ends 

 by a central circular washer (fig. 11). Each spring close to 



Fig. 11. 



© 



the washer bears a peg for the support of the mirror ; and 

 under the point of the spring rises the levelling-screw, by 

 moving which up or down the peg itself is very slightly 

 raised or lowered. These levelling-screws are long and are 

 surrounded below the plate for about half their length by 

 brass tubes (see fig. 7) which when the instrument is in use 

 pass freely through the large holes near the edge of D 5 and 

 dip into annular troughs on the upper side of the lower box 

 of the vapour-jacket (see diagramatic section fig. 12), while 

 the screws themselves pass out through the box. 



Fig. 12. 



The troughs are filled with oil into which the ends of the 

 tubes dip. The object of these oil-joints is to allow of free 

 communication with the exterior, while at the same time 

 preventing any influx of cold air, without impeding the free 

 movement of the disks when the wires are stretched. The 

 weight-supporting hook from the middle of D 5 passes out 

 through a similar oil-joint. (Fig. 12 will make clear the 

 relations of these parts.) It must be observed lhat D 3 and D 4 

 are quite independent of each other, their only connexion being 

 by means of the dash-pots, no metallic contact existing. The 



