682 Prof. J. Joly on the Closure of Small . 



face of one of the hemispheres. The hemispheres may be 

 cemented together by a thin varnish of Canada balsam or 

 simply laid together. The diameter of the spliere so formed 

 is closely 2 cm. It is highly polished on the outside as well 

 as on the flat meeting surfaces. 



The sphere of rock is enclosed in a lead cylinder- having 

 an outside diameter of 2*5 cm. This cylinder, which is about 

 3*4 cm. in length, is composed of two short cylinders in each 

 of which a hemispherical cavity, having the same dimensions 

 as the hemispheres of rock, is formed. The lead cylinder 

 containing the sphere of rock is pushed into the crushing 

 mortar, and exposed to pressure transmitted through a 

 plunger. Tn this manner a hydrostatic pressure acts upon 

 the sphere of rock, the lead flowing freely. The arrangements 

 permitted of the pressure being maintained practically con- 

 stant for several months at a time. 



The crushing mortar is shown in section in fig. 1, 

 to a scale of one half. It is made of vanadium steel, the 

 outer strengthening ring being shrunk on. Messrs. Amsler 

 of Schaffhausen were the makers, and it would be difficult to 

 find fault with the manner in which the apparatus behaved. 

 It will be seen that the lead cylinder is contained between 

 a shorter plunger below and the upper plunger. The latter 

 carries a movable cap resting on a smooth polished surface, 

 to secure as far as possible a truly axial direction for the 

 applied compressive force. This mortar is specified to take 

 on the plunger one hundred tons per square inch. In order 

 to prevent leakage of the lead past the pistons (which fit very 

 accurately) copper washers — turned over on the edge — are 

 provided. One of these is fitted to each end of the lead 

 cylinder. 



The shaping of the rock material to the form of two accu- 

 rately fitting hemispheres was undertaken by Dr. Kran'rz of 

 Bonn. This work was very beautifully and accurately carried 

 out. The internal cavities were ground in the laboratory, 

 using a small spherical steel ball (intended for a ball-bearing) 

 to carry the abradent (very fine carborundum). This ball 

 was mounted in lead, projecting just a very little more than 

 its radial dimension, and spun in the lathe against the central 

 point of the plane surface of the rock hemisphere. The 

 cavity so produced had a diameter of closely 6'2 millimetres. 

 During this operation the rock hemisphere was held in a lead 

 cup, externally cylindrical, in which a cavity of the shape of 

 the hemisphere had previously been formed by a special tool. 

 Special means were employed to secure the central position 

 of the cavity ground in the rock hemisphere. 



