is in the diagram the difference of the two motions; in the reverse inclination, its path would be the sum of the two motions, and i i being a straight line, the correction would be evenly distributed at every part of the length.

Other experimentalists preferred, however, the method of the chain, or flexible band, for traversing the tool the exact quantity; because the reduction of a diameter of the pulley or drum, afforded a very ready means of adjustment for total length; and all the wheels of the mechanism being individually as perfect as they could be made, a near approach to general perfection was naturally anticipated on the first trial. This mode, however, is subject to the error introduced by the elasticity or elongation of the chain or band, and which is at the maximum when the greatest length of chain is uncoiled from the barrel.

About the year 1820, Mr. Clement put in practice a peculiar mode for originating the guide-screw of his screw-lathe, the steps of which plan will be now described.

1. He procured from Scotland some hand-screw tools cut over a hob with concentric grooves; and to prevent the ridges or points. of the screw tools from being cut square across the end, the rest was inclined to compensate for the want of angle in the hob or

cutter.

2. A brass screw was struck by hand, or chased with the tool 1. 3. The screw 2, was fixed at the back of a traversing mandrel, and clipped between two pieces of wood or dies to serve as a guide, whilst,

4. A more perfect guide-screw was cut with a fixed tool, and substituted on the mandrel for 3; as Mr. Clement considered the movement derived from the opposite sides of the one screw, became the mean of the two sides, and corrected any irregularities of angle,

or of drunkenness.

5. A large and a small master-tap m, Fig. 508, were cut on the traversing mandrel with a fixed tool, the threads were about an inch long and situated in the middle of a shaft eight or ten inches long; the small master-tap was of the same diameter as the finished screw, the large master-tap measured at the bottom of the thread the same as the blank cylinder to be screwed. The master-taps m, were used in cutting up the rectangular dies required in the apparatus shown in Fig. 508, and now to be described.

6. On the parallel bed of a lathe were fitted two standards or collar-heads h h', intended to receive the pivots of the screw to be cut, on the extremity of which was placed a winch handle, or sometimes an intermediate socket was interposed between the screw and the winch, to carry the latter to the end of the bed. The bed had also an accurate slide plate s s', running freely upon it, the slide plate had two tails which passed beside the head h', and at the other end, a projection through which was made a transverse rectangular mortise for the dies, the one end of the mortise is shown by the removal of the front die d, and the back die d' is seen in its

proper situation; one extremity of each die was cut from the large master tap m, and the other from the small. The clamp or shackle c c', was used to close the two dies upon the screw simultaneously; it is shown out of its true position in order that the dies and mortise may be seen, but when in use the shackle would be shifted to the right, so as to embrace the dies d d'. The plain extremity c' rested against the back die, whilst the screw c bore against the front die, through the intervention of the washer loosely attached to the clamp to save the teeth from injury; the pressure screw c had a graduated head and an index, to denote how much the dies were closed.

Fig. 508.

7. A cylinder about two feet long, prepared for the screw, was placed between the heads hh', and the large dies, whose inner edges were of the same diameter as the cylinder, were closed upon it moderately tight, and the screw was turned round with the winch, to trace a thread from end to end; this was repeated a few times, the dies being slightly closed between each trip.

8. A screw-tool was next fixed on the slide ss', in a chamfer slide tt', with appropriate adjusting screws, so as to follow the dies and remove a shaving, much the same as in turning. The dies having arrived at one end of the screw, the same screw tool or a second tool was placed on the opposite side of the side-plate so as to cut during the return movement. With the progress of the screw the screw-tool was applied at a variety of distances from the pair of dies, as well as on opposite sides of the screw, so that the metal was cut out by the tool, and the dies were used almost alone to guide the traverse. Of course the dies were closed between each trip, and when the screw was about half cut up the small dies were substituted for the large ones used at the commencement of the process.

9. The screw thus made, which was intended for a slide-rest, was found to be very uniform in its thread, and it was used for some time for the ordinary purposes of turning. When, however, it was required to be used for cutting other screws, it was found objectionable that its rate was nearly nine, whereas it was required to have eight threads per inch. It was then used in cutting a new guide-screw by means of a pair of change wheels of 50 and 56

teeth, which upon calculation were found to effect the conversion with sufficient precision.

10. From 9, the screw of 24 inches in length, one of 8 feet in length was obtained. The thread was cut one-third of its depth, with the wheels, successive portions being operated upon and the tool being carefully adjusted to the termination of the part previously cut. The general truth of the entire length was given by

Fig. 509.

a repetition of the tedious mode of correction represented in the figure, with the dies and tool applied upon a bearer rather exceed· ing the full length of the screw.

Although the processes 7 and 8 will produce a most uniform screw, Mr. Clement attaches little importance to the use of the dies

and guide-frame alone when several screws are wanted strictly of the same length. Of some few thus made as nearly as possible under equal circumstances two screws were found very nearly to agree,

and a third was above a tenth of an inch longer in ten inches. This difference he thinks to have arisen in marking out the threads, from a little variation in the friction of the slide, or a difference in the first penetration of the dies.

The friction of the slide, when sufficient to cause any retardation, he considers to produce a constant and accumulative ef fect; first, as it were, reducing the screw of 15 threads per inch, say to the fineness of 15, then acting upon that of 154, reducing it to 15, and so on; and that to such an extent, as occasionally to place the screw entirely beyond the correctional process. This cannot be the case when the thread is first marked out with the change-wheels instead of the dies.

One very important application of the screw is to the graduation of mathematical scales. The screw is then employed to move a platform, which slides

Fig. 511.

very freely and carries the scale to be graduated; and the swing frame, for the knife or diamond point, is attached to some fixed part of the framing of the machine. Supposing the screw to be absolutely perfect, and to have fifty threads per inch, successive movements of fifty revolutions would move the platform and graduate the scale exactly into true inches; but on close examination some of the graduations will be found to exceed, and others to fall short of, the true inch.

The scales assume, of course, the relative degree of accuracy of the screw employed. No test is more severe; and when these scales are examined by means of two microscopes under a magnifying power of ten or twenty times, the most minute errors become abundantly obvious from the divisions of the scales failing to intersect the cross wires of the instrument; the result clearly indicates corresponding irregularities in the coarseness of the screw at the respective parts of its length. An accustomed eye can thus detect, with the microscope, differences not exceeding the one thirtythousandth part of an inch, the twenty-five-thousandth part being comparatively of easy observation.

Figs. 509 and 512 show a large chucking and reaming lathe built at Lowell, Massachusetts.

Figs. 510 and 511 show a chucking and reaming lathe manufactured at Lowell. This instrument is geared with a rest for holding drills and reamers moved by a toothed rack, backhead

stock, adjustable sideways, cast-iron cone-pulleys, gun metal bearings, and cast-steel spindle.

Fig. 513 is an engine lathe manufactured at the Lowell Machine Shop, Lowell, Massachusetts. Its swing is 50 inches over the sills, and 32 over the rest.

The bed of this lathe is cast in one piece, the feed motion is carried by a screw, the tool rest held down by gibs under the slides, and moved on a toothed rack and pinion by hand.

SCREW THREADS CONSIDERED IN RESPECT TO THEIR PROPORTIONS, FORMS, AND GENERAL CHARACTERS.-The proportions given to screws employed for attaching together the different parts of works are in nearly every case arbitrary, or, in other words, they are determined almost by experience alone rather than by rule, and with little or no aid from calculation, as will be shown.

In addition to the ordinary binding screws, which, although arbitrary, assume proportions not far distant from a general average, many screws, either much coarser or finer than usual, are continu