Top Edge.Bottom Edge.
Contrac-
tion.		Percentage
of original
circum-
ference.		Expan-
sion.		Percentage
of original
circum-
ference.
Inches. Inches.
After 1st cooling .01.05.05.26
„2nd„ .01.05.08.42
„3rd„ .02.10.14.75
„4th„ .02.10.17.90
„5th„ - No change from
original size
from 5th to
20th cooling.		.221.17
„10th„.402.13
„15th„.563.00
„20th„.703.70

“It will be unnecessary to occupy much time in analyzing the experiments, as any one who takes a practical interest in the subject will have full information in the diagrams and tables. Professor Stokes drew attention to the fact that, in 1863, similar phenomena had been noticed by Colonel Clark, of the Royal Engineers. His experiments, made at the Royal Arsenal, Woolwich, were published in the ‘Proceedings of the Royal Society,’ and Professor Stokes had himself attached an explanatory note, the outline of which was as follows:—

“Imagine a cylinder divided into two parts by a horizontal plane at the water-line, and in this state immersed after heating. The under part, being in contact with water, would rapidly cool and contract, while the upper part would cool but slowly. Consequently by the time the under part had pretty well cooled, the upper part would be left jutting out; but when both parts had cooled their diameters would again agree. Now in the actual experiments the independent motion of the two parts is impossible on account of the continuity of the metal; the under part tends to pull in the upper, and the upper to pull out the under. In this contest the cooler metal, being the stronger, prevails, and so the upper part gets pulled in a little above the water-line while still hot. But it has still to contract in cooling, and this it will do to the full extent due to its temperature, except in so far as it may be prevented by its connection with the rest. Hence, on the whole, the effect of this cause is to leave a permanent contraction a little above the water-line, and it is easy to see that the contraction must be so much nearer to the water-line as the thickness of the metal is less, the other dimensions of the hollow cylinder and the nature of the metal being given. When the hollow cylinder is very short, so as to be reduced to a mere hoop, the same cause operates, but there is not room for more than a general inclination of the surface, leaving the hoop bevelled.

“The expansion of the bottom edge was not noticed in Colonel Clark’s paper, perhaps owing to the much smaller hoops which he used in experimenting. Accepting Professor Stokes’ explanation of the top contraction, it appears that expansion of the bottom may be accounted for by the reacting strain put on the cooled edge when forcing in the top edge, acting in such a way as to prevent the cooled edge coming quite to its natural contraction, and this, when sufficiently great, expresses itself in the form of a slight expansion.

“Experiment No. 14.—Forged steel hoop, turned and bored, 18.53 inches in circumference outside by 2.375 inches deep by .27 inch thick, heated to redness, then cooled one-half its depth in water (see [Fig. 1437] at g for final form of hoop after three heatings and coolings).

Top Edge.Bottom Edge.
Contrac-
tion.		Percentage
of original
length.		Expan-
sion.		Percentage
of original
length.
Inches. Inches.
Afer 1st cooling.06.32——{Cracked at water‑cooled
edge one‑third depth of ring.
„2nd„.12.64——
„3rd„.201.08.05.27{After allowing for three small
cracks in bottom edge.”

The shrinkage of iron and steel by cooling rapidly is sometimes taken advantage of by workmen to refit work, the principles involved in the process being as follows:—