I. SPECIAL COLLECTIONS. 41 



215. Measuring Instrument for the Bores of Guns. 



Differences of one ten thousandth of an inch can be measured with 

 this instrument. 



This instrument was designed by Sir Joseph Whitworth to measure the 

 expansions of the bore of a gun, due to the explosion of the charge of powder, 

 and to ascertain what strain a gun would stand before permanent alteration 

 took place. 



The instrument is moved by hand along the bore of the gun, and the pro- 

 jecting feelers are connected to a micrometer at the end of the bar. 



Differences of one ten thousandth of an inch are quite perceptible. 



216. Standard Screw Gauge of Whitworth thread. 



This gauge is to show the form of the Whitworth standard thread. The 

 two cylindrical parts give the exact diameter of the top and bottom of the 

 screw threads in universal use ; the angle is 55 rounded off |th of its 

 depth. 



217. New Hexagonal Surface Plates designed to insure 

 that the surfaces remain true under different conditions. 



The original surface plate first exhibited by Sir Joseph Whitworth at 

 Glasgow in 1840 was rectangular, and ribbed on the under side in such a way 

 as to allow of being supported on three points. 



Two handles, one at each end, were fastened to the plate for lifting, and 

 from them it was suspended when about to be applied to the work in 

 hand. 



When rectangular surface plates of large size Avere suspended a straining 

 took place, and they were no longer as true as when supported on the three 

 points. The new hexagonal surface plate, besides being supported on three 

 points, is also suspended from the same three points, and remains true when so 

 suspended. ^ 



The surface remains true under different conditions: a 30-inch plate was 

 heated to 200 degrees, and at that temperature did not vary the 1,000th of an 

 inch. 



The true plane is the foundation and source of all truth in mechanism. 



APPARATUS USED BY DR. JOULE, F.R.S., FOR ASCERTAINING THE 

 MECHANICAL EQUIVALENT OF HEAT. 



218. Revolving Electro-magnet, used in 1843 for ascer- 

 taining the Mechanical equivalent of Heat. 



Part of the apparatus used in 1843 for the determination 'of the mechanical 

 equivalent of heat : viz., a revolving piece, holding a glass tube filled with 

 water, and containing an electro-magnet. This worked between the poles of 

 a powerful magnet ; and the heat evolved by the rotating electro-magnet was 

 measured by the rise of temperature of the water. In this manner the quan- 

 tity of heat lost by the surface was ascertained when the machine worked as 

 an engine ; and, on the other hand, the quantity of heat produced when work 

 was done on the machine was also measured ; 833 ft. Ibs. was the mechanical 

 equivalent of a degree Fahr. in 1 Ib. of water, as determined by these first 

 experiments. 



219. Calorimeter, containing a revolving agitator. This 

 was employed in the experiments on the heat evolved by the 

 friction of water, made in 1849. The equivalent arrived at was 

 772 ft. Ib. 



