ox THE MECHANICAL EQUIVALENT OF HEAT. . 
343 
(2.) A brass bar by Elliott and Co., 39 inches long, and graduated in inches, 
used as the standard in the physical department in Owens College. 
From the Whitworth gauges, two steel bars, f inch in diameter and 9 inches long, 
with parallel plane ends, were made by Mr. Foster, and compared with the 9 -inch 
Whitworth bar by the measuring machine. 
With these and the Whitworth gauges, placed end to end, an outside gauge con¬ 
sisting of two surfaced angle-plates on a surfaced cast-iron bed was set out, and then 
a steel bar | inch in diameter with plane ends fitted to these. Careful com¬ 
parison showed that this bar did not differ from the sum of the lengths of the gauges 
by T o ooo parts of an inch. This length was then carefully laid off by the surfaced 
angle plates on the surface plate, and was so compared with the scale of the 
Elliott brass bar, account being taken of the temperature, and found to agree within 
less than yq -^ qo of an inch. 
The 30-inch bar so obtained was then taken as the standard both for the levers of 
the brake and the barometer, to be carefully preserved. 
Lengths of the Levers. 
45. The V-groove, in which the knife-edge of the carrier, by which the load on the 
brake was suspended, rested, was originally made at a distance of four feet from 
the axis of the shaft at ordinary temperatures, and, as whatever the error might be 
when the brakes were hot, it would be the same for all the trials, since the tempera¬ 
tures were the same, it was decided to take this as the length of the levers in 
estimating the loads during the progress of the research, and to treat whatever error 
there might be as a standing correction on the final results. Such correction to be 
obtained by laying off four feet Jess the radius of the shaft from the carefully squared 
end of a steel plate 3 inches broad, inch thick, then placing this, flat, in a vertical 
plane perpendicular to tlie shaft, with its edge horizontal, as near as practicable to the 
knife-edge groove with the squared end touching the shaft. Then by means of a 
theodolite, set so that its line of collimation was in a vertical plane parallel to the 
axis of the shaft, and intersecting the vertical line on the plate, to observe the 
distance of the groove from the line on the plate, while the brake was running under 
the same conditions of temperature, and load as in the trials ; but with the carrier 
temporarily displaced further along the shaft, so as to leave the bottom of the 
V-groove visible through the theodolite and in this way to obtain the actual 
distance of the groove from the axis of the shaft as affected by the ex|3ansion of 
the brake and any displacement of the bearing on the shaft which might result from 
the running. 
By using a scale divided to the oiie-hundredth of an inch, and taking several read¬ 
ings, this could be determined to a thousandth of an inch, so that the limits of 
accuracy would be 
± 0 - 00002 . 
