EFFECT OF PRESSURE UPON ARC SPECTRA. 
115 
The Carriages for the Grating and Camera. (Fig. 3.) 
Each carriage consists essentially of a heavy triangular casting supported upon 
three wheels, two of which are grooved to run upon the rail of the girder, while the 
third is flat. The whole is heavy, and stability is assured by the distance apart oi 
the grooved wheels, which is 15 inches. 
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i_ i ■ ■ ■ i i INCHES 
Fig. 3. The carriages for grating and camera. 
Each end of the cross-beam, fig. 2, rests upon a casting (figs. 4 and 5, A), which 
turns upon a raised annular bearing surface (fig. 3, B) on the top of the carriage, and 
which has a solid steel pin, C, figs. 4, 5, to fit in a vertical hole drilled in the carriage 
itself. A cast-iron plate (F, fig. 4, E, fig. 5) furnished with a collar fitting over the 
top of this casting, to which it is firmly fixed by means of a set-screw, forms a table 
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