The three bits are simultaneously and equally advanced by the central wedge i, shown also detached, which resembles a cone, reduced so as to form three fins at right angles to one another, that enter the grooves for the bits; the wedge i, is set forward by the tail-screw k, there is a side screw, which prevents the unintended movement of the set screw k, to arrive at which latter it is necessary to remove the socket c g, from the stem of the instrument g h.

If the wedge with its three fins were considerably advanced beyond its present position, it would stand before the cutter, and prevent the tool from proceeding to the extremity of a flat-bottomed hole: in order to avoid this, Mr. Kittoe has made the back edges of the cutters at the same angle as the wedge, so that the act of setting forward the wedge also sets forward the cutters to keep them in advance of the point of the wedge. The cutters have oblique mortises which pass over the retaining pins fixed in the semicircular piece, and prevent the cutters from being accidentally lost.

It is the intention that every instrument constructed on this mode should possess various sets of the bits, a b, so proportioned in size that the smallest instrument should be capable of being expanded to the smallest size of the second instrument, the second to the third, and so on, in order that a few of the boring bars, or stocks, may serve for a considerable range of sizes.

Note BJ. - To follow foot note on page 572. (Mr. George Wright, inventor of the modem system of boring large Cylinders.)

The author is informed by John Taylor, Esq., F.R.S., etc, that the mode of boring steam engine cylinders, by means of a revolving bar with a traversing cutter block and cutters, as slightly explained in fig. 517, page 571, was invented by Mr. George Wright, whilst in the employment of Messrs. Boulton & Watt, of the celebrated Soho Works, near Birmingham. This admirable contrivance has proved of immense advantage to practical engineers.

It may be added that Mr. Nasmyth constructs his heaviest boring bars with three longitudinal grooves fitted with adjustable chamfer bars, and that sustain the pressure of the cut against those faces of the chamfer bars which are radial to the bar. The vertical position seems now to be rather preferred to the horizontal for this class of boring machines, so that the cylinders may be bored in the position in which they are afterwards erected.

Note BK. - To follow the text on page 580. (Mr. Mallet's method of describing regular and irregular spirals.) We transcribe from the "Mechanics' Magazine" for 1844, page 55, the commencement of a very useful paper by Mr. Mallett of Dublin: -

"For many purposes of the arts, a simple and rapid method of tracing spirals upon a cylindric surface is important; carvers, wood-turners, etc, often want such, and in larger works, such as some particular branches of mill-work and engineering, it is also frequently wanted. The usual method, by dividing the cylindric surface into equal portions in circumference and length, and drawing lines diagonally, is tedious."

"The following method, bettered to be new, is simple and ready, and sufficiently exact for most purposes. Two straight edges of equal length and width, and about 5/8ths of an inch in thickness each, are to be secured on a table parallel to each other, standing on their edges, and distant from each other by nearly the length of the cylinder upon which the spiral is to be marked. Between these there is also to be secured, in a diagonal direction, stretching from one to the other, a third straight edge, formed of two slips of deal glued together, with a slip of straight thick Bristol board between them projecting 1/8th of an inch at one edge."

"The entire height of the diagonal straight edge when standing on the table, must be a shade more than that of the two other straight edges. The three pieces being then thus arranged, the edge of Bristol board is charged with printer's ink. Then, on causing the cylinder to roll over the edges of the two parallel straight edges in the direction of their length, the diagonal slip of inked Bristol board will trace a spiral upon the surface of the cylinder with very considerable accuracy."

Mr. Mallett then goes on to describe that by substituting a curved edge for the inclined straight edges variable screws will be described, following any particular condition set out in the developed surface of the screw as represented by the curve: this he considers useful in setting out the variable screws or those of increasing pitch for propellers, and he further shows that spiral lines may be thus drawn on cones, prisms, etc.

Note BL, page 696, to follow the paragraph ending " a convenient bed for the file." (On Sharpening the teeth of Saws by meant of Grindstones.)

A peculiar mode of sharpening the teeth of large circular saws by means of grindstones, the author is informed, is followed by Mr. James Boag, of Johnston, Scotland, manufacturer of Casks and various works in wood.

A small grindstone, mounted on a spindle, and turned on the edge with a narrow ridge suited to the form of the teeth, is made to revolve by the steam engine; the circular saw is placed upon the surface of a slide, having a center pin to fit the axis of the saw, and a stop to determine now nearly it shall approach the grindstone: the platform or slide is inclined agreeably to the angle at which the stone should meet the saw plate, and there is a detent or hook, which by catching against one of the teeth, holds the saw plate in the positions successively required for every other tooth around its circumference.

The grindstone from its rapidity of action is constantly employed when much has to be removed, as in depthening the gullets, when but little is required to be done to the saw, the Ale is employed as usual.

Sometimes also the saw remains at rest except as regards the change from tooth to tooth, and the grindstone is mounted on a swing frame and brought down every time to a stop.