OF ARTS AND SCIENCES. 



107 



TABLE v.- 



Comparison of the Spiral Gadge with toe Bourdon 

 Gauge. 



Figure 8, which exhibits these data graphically, shows that between 

 zero and 300 atm. the permanent set is not appreciable. When the 

 pressure is taken off, the fiducial zero reappears, Above 300 atm., 

 permanent set becomes very marked. The helix was ruptured at 

 1,000 atm. Connecting the final zero (after rupture) with the point 

 for 900 atm., a line is obtained more nearly parallel to the line be- 

 tween zero and 300 atm. The mean motion of the index was, there- 

 fore, .0017 cm. per atm. Since high pressure measurement is only 

 contemplated, the gauge would have been serviceable except for the 

 occurrence of permanent set. The cause of this insuperable difficulty 

 is the forging necessary to flatten and bend it, for the drawn resilience 

 of the tube is thus destroyed.* Clearly, a cold bent tube might be 

 used without flattening by suitably attaching a mirror index ; but this 

 complicates the gauge, and, owing to the great difficulty of working 

 steel, I abandoned further attempts. 



22. Concluding liemarks. — Instead of having the screw enter the 

 barrel, good results must also be obtainable by forcing a cylindrical 

 shaft into the barrel. The device shown within CO, Figure 1, proves 

 that the gasket of marine glue is quite as serviceable for smooth cylin- 

 ders as for screws. In case of a cylindrical plunger, however, the 

 strain encountered in forcing the plunger home would be brought to 

 bear on the bedplate. Supposing the general disposition to be re- 

 tained, this would make the apparatus more cumbersome ; for specially 

 strong devices would have to be resorted to in bolting down the barrel, 

 etc. Indeed, I mentioned it as an advantage of Andrews's screw, that 



* To make tlie steel Bourdon, it would be necessary to draw flat steel tubes. 



