RESEARCHES ON THE CONTORTION OF ROCKS. 
25 
bending layers of cloth and clay. The omission may well seem 
strange to us, but an explanation is to be found in Hutton’s 
teaching. Uniformitarian as he was, Hutton did not believe 
that great internal changes had taken place in strata after their 
solidification ; nor did his pupil, when seeking to establish the 
Huttonian theory by direct experiment, think it needful to 
investigate the properties of consolidated rock. 
In 1866 I began some simple experiments, taking up the 
points that had been disregarded by Hall. First of all, I took 
a thin slab of marble and placed it on the edge of a mantel- 
piece, so that the end projected. A few books kept the slab in 
its place and then I placed a letter weight of one ounce on the 
free end and left it for some weeks. On testing it by a straight 
edge it was found to be deflected to a trifling extent. Other 
plates of different materials, two or three inches long and as 
thin as possible, were next procured and subjected to the same 
treatment. But no accurate results were obtained, and the 
form of the experiment was inconvenient. 
Subsequently I tried an improved plan. Two wooden slabs, 
ten' inches by four, were fitted together by a hinge so that they 
could be set at any angle from 0° to 180°;, just as you might 
open a book, keeping the letterpress always downwards. One 
slab was screwed to the table, the other could be adjusted at 
pleasure. The angle made by the two surfaces was indicated 
by a graduated semicircle. Upon the ridge various thin plates 
of stone were placed and attached at one end to the fixed slab 
by heavy weights. The other end of the plate of rock, project- 
ing over, but not at first in contact with the movable slab of 
wood, was lightly weighted and allowed time for bending. 
The angle of sudden fracture could be obtained by setting the 
machine at a low angle and forcibly bending down the lamina 
of rock until it touched both surfaces. If it yielded thus far, 
the angle was slightly increased and the experiment was 
repeated. 
This apparatus had some advantages but many defects. The 
most serious was that the pressure exerted at any time was dif- 
ficult to estimate. A weight placed upon a surface of gradu- 
ally increasing inclination exerts a diminishing pressure which 
changes appreciably even at angles of 3° or 4°. It was difficult 
to read the small deflections obtained with any accuracy, and 
the apparatus was liable to disturbance and accident. 
At length I tried the machine represented in Plate LXXX.* 
Here a thin plate of limestone or other rock is screwed down 
to a block / travelling in a horizontal groove h. Upon this 
* For the construction of this machine, and for many useful suggestions, I 
am indebted to Mr. Thomas Prince, of Bradford. 
