It consists of an iron tube, which passes through an aperture on the upper joint of the guiding-frame, into which is screwed a round piece of gun-metal, slightly hollowed in the centre, but flat towards the edge. This gun-metal disc is perforated by a small hole communicating with the interior of the iron tube. This aperture permits the air between the glass plate and the chuck to be exhausted by a small air syringe at the other end. The face of this chuck is covered with a thin film of soft India-rubber not vulcanized, also perforated with a small central aperture. When the chuck is properly adjusted, and the India-rubber carefully stretched over the face of the gun-metal, one or two pulls of the syringe-piston is quite sufficient to hold a very large object to the action of the slitting-plate. By this method no time is lost; the adhesion is made instantaneously, and as quickly broken by opening a small screw, to admit air between the glass-plate and the chuck, when the object is immediately released. Care must be taken, in stretching the India-rubber over the face of the chuck, to make it very equal in its distribution, and as thin as consistent with strength. When this material is obtained from the shops, it presents a series of slight grooves, and is rather hard for our purpose. It ought, therefore, to be slightly heated, which renders it soft and pliant, and in this state should now be stretched over the chuck, and a piece of soft copper wire tied round it, a slight groove being cut in the periphery of the chuck, to detain the wire in its place. When by use the surface of the India-rubber becomes flat, smooth, and free from the grooves which at first mar its usefulness, a specimen may be slit of many square inches, without resort being had to another exhaustion by the syringe.

But when a large, hard, siliceous object has to be slit, it is well for the sake of safety to try the syringe piston, and observe if it return forcibly to the bottom of the cylinder, which proves the good condition of the vacuum of the chuck.

After the operation of slitting, the plate must be removed from the spindle of the lathe, and the flat lead lap substituted. The pneumatic chuck is now to be reversed, and the specimen placed in contact with the grinder. By giving a slightly tortuous motion to the specimen, that is, using the motion of the

various joints, the object is ground perfectly flat when the length of both arms of the joints are perfectly equal. Should the leg of the first joint on the right-hand side be the longer, the specimen will be ground hollow; if shorter, it will be ground convex. But if, as before stated, they be of equal length, a perfectly parallel surface will be obtained.

In operating on siliceous objects, I have found soap and water quite as speedy and efficacious as oil, which is generally used; while calcareous fossils must be slit by a solution of common soda in water. This solution of soda, if made too strong, softens the India-rubber on the face of the pneumatic chuck, and renders a new piece necessary; but if care be taken to keep the solution of moderate strength, one piece of Indiarubber may last for six months. The thinner and flatter it becomes, the better hold the glass takes, unless a puncture occur in the outer portion, and a new piece is rendered necessary.

Before concluding, I must warn the amateur lapidary against the belief that all hard stones are equally easily slit by diamond powder. As a general rule, the hardest stones are most easily slit (this does not, however, include calcareous ones); but some fossils on which I have operated, though not so hard as others, have completely resisted the action of the diamond powder. For instance, the Yu stone of China, which is by no means so hard as corundum, is much more difficult to slit, and consumes an amount of diamond powder which renders it the horror of the lapidary. This peculiarity is easily understood. If, for instance, we should attempt to saw lead or copper with our diamond slitting-plate, we should find that the diamond powder becomes thoroughly impacted into these metals. In the same way the diamond is taken out of the plate by a soft tenacious fossil, and is impacted in the stone, and no work is accomplished. The method of operating on such specimens is to use emery by the usual method, by which much more speed will be obtained.

The polishing of the section is the last operation. This is performed in various ways, according to the material of which the object is composed. If siliceous, a lap of tin is to be

used, about the same size as the grinding lap. Having turned the face smooth and flat, a series of very fine notches are to be made all over the surface. This is accomplished by holding the edge of an old dinner-knife almost perpendicular to the surface of the lap while rotating; so as to produce a series of criddles, or slight asperities, which detain the polishing substance.

The polishing substance used on the tin lap is technically called Lapidaries' Rot-Stone, and is applied by slightly moistening the mass, and pressing it firmly against the polisher, care being taken to scrape off the outer surface, which often contains grit. The specimen is then to be pressed with some degree of force against the revolving tin lap or polisher, carefully changing the plane of action, by moving the specimen in various directions over the surface.

To polish calcareous objects, another method must be adopted as follows:

G

A lap or disc of willow wood, three inches in thickness, and about the diameter of the other laps (10 inches), is to be adapted to the spindle of the lathe, the axis of the wood being pa

rallel to the spindle of the lathe, that is, the acting surface of the wood is the end of the fibres, or transverse section.

This polisher must be turned quite flat and smoothed by a plane, as the willow, from its softness, is peculiarly difficult to turn. It is also of consequence to remark, that both sides be turned so as that the lap, when dry, is quite parallel. This lap is most conveniently adapted to the common face chuck of a lathe by a conical screw, so that either surface may be used. This is made evident, when we state that this polisher is always used moist, and that to keep both surfaces parallel, it must be entirely plunged in water before using, as both surfaces must be equally moist, otherwise the dry will be concave, and the moist surface convex. The polishing substance used with this lap is putty powder (oxide of tin), which ought to be well washed, to free it from grit. The calcareous fossils being finely ground, are speedily polished by this method. To polish softer substances, a piece of cloth may be spread over the wooden lap, and finely-levigated chalk used as a polishing agent.

Remarks on the Gyroscope, in relation to his "Suggestion of a New Experiment which would demonstrate the Rotation of the Earth." By EDWARD SANG, F.R.S.E.*

At its tenth meeting of last Session (9th April 1855) the Society was favoured by Mr Elliot with an exposition of the phenomena of rotation, in the course of which he exhibited his beautiful method of producing the disturbance of the axis by means of magnetic attractions and repulsions.

In that exposition, Mr Elliot also gave an account of the experiments which had been performed by M. Foucault at the meeting of the British Association in Liverpool; and he seemed inclined to give the merit of the discovery of the various phenomena to the illustrous Frenchman.

I immediately took the opportunity of stating that, with one exception, the phenomena had been regularly exhibited and

* Read before the Society, 24th March 1856.

explained by the late Sir John Leslie, during his Lectures on Natural Philosophy, not as at all new, but as well known and well understood results of the acknowledged laws of Mechanics.

The only variety of the phenomena which he had not exhibited is that which refers to the motion of the earth; and I took the liberty of reminding the Society that, many years before, I had proposed that experiment, and illustrated the manner of carrying it out, by the very apparatus which had been used by Leslie, while, at the same time, I had exhibited the previously known experiments.

Several of my friends urged me to lay my claims to priority formally before the public. As I happened to be prosecuting some investigations in which the matter is naturally involved, I delayed until these investigations should have been completed; but the crowd of subjects which, after an absence of thirteen years, all at once claimed my attention, have prevented me from maturing these investigations; and, being unwilling to let another Session pass without formally asserting my claim, I have now to beg the attention of the Society to the general history of the matter.

As soon as the precession of the equinoxes was traced to the attractions which the moon and sun exert upon the protruded portions of the earth, the idea of illustrating this phenomenon by mechanical appliances must have arisen, the more so that the common spinning-top exhibits the recession of the nodes, and that the two phenomena are identical in theory.

Hence the precession machine is a very old instrument. I exhibit one that was presented by M. Arago to Professor Playfair, and which must have existed since 1816-1817. It is the type of most that have been made since. The instrument which belonged to Leslie, and which I exhibited before you twenty years ago, was slightly larger, and much better made. And that one which was procured by Professor Forbes to supply the want of Leslie's, is, I am told, just such another. I am, then, justified in stating that, ever since the year 1817, the apparatus for exhibiting these phenomena has not been wanting in our University; and I feel confident that the full