1130 
MU. G. F. ROD WELL OH THE EFFECTS OF HEAT OH CERTAIH 
conduction of heat from the short glass rods (E and H) to the interior of the apparatus. 
The coefficient of expansion of E and H must also be ascertained, and allowed for in 
all determinations. If, on the other hand, we wish to determine the contraction of a 
body, the index is pushed up to 180° of arc by means of the micrometer-screw, and is 
pushed back to zero as the bar contracts by the spring Y and the helical mainspring Z. 
The precise value of the return movement must be, of course, ascertained by means of 
the micrometer-head. It is obvious that in moving from 0° to 180° of arc the spring 
Y acts against the motion of the expanding body; while in moving from 180° to 0° it 
moves with the motion of the contracting body. All the earlier determinations were 
made with the apparatus in this form. 
Subsequently, and for the later determinations, the following changes were made 
mainly with a view of reducing the resistance by diminishing friction 
1. The wooden base N was replaced by a massive stone block, to which the box 
containing the levers, and the upright carrying the micrometer-head were firmly bolted. 
2. The levers S, U (fig. 2) were reduced in weight. 
3. The spring Y (fig. 2) was removed, as it was found that the recoil of the helical 
spring Z was quite sufficient to bring the index back to zero, and the presence of Y 
served only to increase the resistance and general strain. 
4. The vertical axis X (figs. 1 and 3) was no longer allowed to work in the upper 
confining plate of the framework b, but it was caused to turn lightly in a bent arm 
above, while below it rested upon a slightly hollowed ruby. 
5. The steel chain W communicating motion from the lever U to the vertical axis 
X, which carries the index, was shortened and caused to wind upon the barrel in such 
a manner that when unwound to the extent of half a single coil, it moved the index 
through its entire range, viz.: from 0° to 180° of arc. By this means any possibility 
of the chain doubling upon itself was obviated. 
6. But perhaps the most important changes were the removal of the stuffing boxes I 
and the rods H from the trough F, and the substitution of levers working over the 
rim of the trough ; and the suspension of the rod of substance under examination in 
a cradle between the levers. This was effected in the following manner :— 
A horizontal bar L, L (fig. 4) was supported by rods N, N, strengthened by cross 
bars (not shown) let into the stone base of the instrument; it carried Y-shaped brass 
levers B, B, moving about axes at C, C (figs. 4 and 5), attached at the points A, A. 
F is the trough in which hot ceresine is used for heating the bar under examination, 
H, which is supported by the cradle G. Two rods I, I, which slide in holes K, K, 
and are capable of being held at any height by screws, support the cradle G. D is 
the rod (figs. 1 and 2) which bears upon the lever S, and M the point of the micro¬ 
meter-screw. 
A bar of fine homogeneous silver was used hi order to test the delicacy of the 
apparatus. It was placed between the levers B, B, and cold water was poured into 
the trough. The temperature was indicated by two thermometers reading well 
together, placed near the opposite extremities of the silver bar, and the needle was 
