With this arrangement of apparatus, Mr. Hodgson succeeded without material difficulty in grinding and polishing specula, one of which is 3 1/2 inches aperture, with a focal length of 33 inches; this has a tolerably good figure, and performs very well.

In figuring this speculum an elliptical polisher was used, the proportions of which were the same as those recommended by Mr. Edwards (see page 1278). To allow of the free rotation of the polisher, which was made of a mixture of lead and tin, the upper part was finished as a cylinder, to fit loosely in the ring, and the length of traverse of the center of the polisher across that of the speculum, was rather more than one inch.

Mr. Hodgson suggests that should it be considered desirable, a second guide pulley may be placed at the back, instead of the steel spring, and a second eccentric on the spindle of the overhead motion would, no doubt, answer quite as well to produce the back stroke; but the plan which he followed appeared in his own case to be more easily executed.

A machine employed at the Vauxhall pottery works, for grinding the spherical stoppers of air-tight earthenware jars, is represented in the diagram, fig. 1138. The stoppers and jars Fig. 1138 were ground together in the state in which they left the kiln, without separate preparation. About a dozen jars were fixed by clamping apparatus around the margin of a circular table, that was suspended by swing chains from the upper part of the frame of the machine. The circular table was swung bodily in a circle of about three inches diameter, by a slowly revolving eccentric placed beneath, and every stopper was made to revolve with considerable rapidity within the spherical fitting of the jar by the following arrangement.

The Production of Spherical Surfaces By Abrasion P 30082

A large toothed wheel fixed horizontally in the center of the upper part of the frame, communicated by a pair of bevil wheels with a horizontal shaft driven by a strap from the engine. Around the central wheel about a dozen small pinions were mounted in separate bearings at equal distances, so as to be all driven at the same time by the central wheel. The axis of every pinion passed through its bearing, and terminated beneath in an eye to which a hooked rod was suspended. To allow of variation in the length, this rod was fitted within a piece of gas-tube with a slit down the side for a pin to ensure the rotation of the tube, which terminated at its lower extremity in a chuck for the attachment of the stopper.

The rapid revolution of the stoppers gave the grinding motion, and the slow circular swinging of the table derived from the eccentric beneath, gave the continual change of position required for the true spherical form. The weight of the tube and chuck supplied the pressure for the grinding, which was begun with sand and water, and completed with emery, the time occupied in grinding a dozen jars being about one hour. This machine was perfectly successful in producing the true spherical fitting, but is now little used, as it is found more economical to grind the stoppers and jars together in the lathe by hand, the true spherical form not being considered by manufacturers of sufficient importance to justify the additional expense of the machine.