SCUKW. 



;\v MANTI AcrriiK. 





Ue handle of a winch, or to a (tring passing over UM circumference of 

 puller attached to the rod. The threads work between teeth on the 

 ^iterance of a wheel, *o that while the revolution of the rod con- 

 tinue*. the wheel turns on iU own axis. If the radius of the winch 

 or of the pulley on the Mrew-rod be 6 inch*, and the distance between 

 the thread* of the tcrew be l-10th inch, a power represented by unity 

 at the cireuuiferenoa of the pulley will be in equilibrio. uuiittiiig the 

 effects of friction, with a resistance expressed by x 10 x 2, that U, 

 by 370-99 (w being the half circumference of a circle whoae radius is 

 unity) applied at the thread* of the crew or at the circumference of 

 the wheel 



If the line* A r, c u, ftc., were drawn on paper, and that paper were 

 bent on the surface of a convex or concave cylinder, one spiral thread 

 would be formed by the union of A r, E K. &c., at their extremities, 

 and another by the like union of f n. o u, *c. : thin is called a double- 

 threaded acrew, and it in evident that its mechanical power depends 

 upon the ratio of A 8 to u r, while that of the single-threaded screw 

 depend* on the ratio of A * to B D ; that a, with apparently an equal 

 distance between the thread* on both screws, the power of the latter 

 is double that of the former. 



From the high ratio which the resistance bears to the moving-power 

 in the screw, the use of this machine for moving or compressing bodies 

 U very great ; it is also extensively employed in the construction of 

 philosophical instruments for measuring small angles or distances. 

 [MICROMETER,] 



The ingenious screw-machine which was invented by Mr. Hunter, 

 and is described in the ' Phil. Trans.' vol. 17, consists of one convex 

 screw which work* in the interior of another convex screw. 

 The Utter works in a concave screw which U fixed; and the 

 former is capable of moving in a rectilinear direction only, being pre- 

 vented from turning on its axis with the rotation of the exterior 

 screw. Also the number of threads in an inch on the convex surface 

 of thi* last is less by one than the number in an inch on the convex 

 surface of the other : suppose the first number to be 10 and the 

 other to be 11 ; then one revolution of the exterior screw would 

 cause the whole machine to move forward through a space equal to 

 h inch, but in the same time the interior screw is carried backward 

 through ("j inch. Therefore the forward motion of this last is equal 

 to ]5j ,' v , or jjg inch ; and to produce an equal effect in a simple screw, 

 the Utter should have 110 threads in an inch. 



Mr. Barlow of Woolwich mentions a compound machine of this 

 kind in which the exterior screw had 100 threads in an inch, and the 

 interior screw 101 threads ; -therefore one turn of the machine caused 

 the latter to move through the very minute extent of ,'-, inch, and 

 this space was further subdivided into hundredths by means of a 

 micrometer head applied to the exterior screw. 



The endless screw is a component part of graduating machines, 

 counting machines, tc. ; it is also employed in conjunction with a 

 wheel and axle to raise heavy weights. 



Sfrtm, or Spiral, of Archimedes. The inventor of the machine so 

 called is quite unknown, but both Diodorus Siculus and Athemeus 

 ascribe the origin of it to the philosopher of Syracuse. The former 

 relate* (' Bibl. Hist.', lib. i., c. 34) that irrigation was facilitated in 

 Egypt by a certain machine invented by Archimedes of Syracuse, and 

 called Cochlias (m*xAlat) from its form ; and the latter states 

 (' Deipoosophistoo,' lib. v., p. 208, Casaub.) that Archimedes invented it 

 for the purpose o( removing the water from the hold of the great ship 

 which was built by King Hiero of Syracuse. Vitruvius (' lie Archi- 

 tecture,' lib. x., c. 11) describes the machine under the name of 

 cochlea. He says that it consisted of four or eight laths beut spirally, 

 and fixed at one edge against the axle, so as to form as many winding 

 channel* about it ; and that the whole was covered by a cylindrical 

 case, formed of planks, nailed over the exterior edges of the laths. 

 The lower extremity was immersed in the water, which, rising along 

 the channels by the i . solution of the machine on its axis, was dis- 

 charged at the upper extremity. Vitruvius adds that it was turned 

 by men walking on its outer circumference, probably on the conical 

 surface of a bevelled wheel fixed to the axle. 



By the account which Vitruvius has given of its disposition, the 

 spiral laths were placed nearly at an angle of 45 with the axle, and 



the Utter wai inclined to the horizon in an angle of 30 52'. The 



cause of the ascent of the water, and tho limits of thu inclination of 

 the axle to the horizon, will be seen from the following details which 

 the modern form of Archimedei's spiral as it is also called. It 

 may consist of either a flexible tube open at both i-nd^ ..n.l wound 

 spirally on the exterior surface of a cylinder ; or it may be a plate of 

 metal coiled about an axis, like the threads of a screw, and enclosed 

 within a hollow cylinder so as to be complet. ly v.^tci --tight . 

 machine U fixed in an inclined position, with its lower extremity 

 immersed in the water which is to be raised. While it is at rest the 

 water occupies the lower part between two of the threads or beu<l.- of 

 the spiral, at bottom ; but, when turned on its axis, this port of the 

 machine being made to ascend, the water will by its gravity be caused 

 to descend into the lower part between the next bends of the spiral, 

 while in reality it rises, with respect to its former position, in con- 

 sequence of the rotation of tho tube, or bends, within which 

 confined. Thus the water continually proceeds towards the up|r |>art 

 of the machine, from whence it is discharged into a reservoir placed to 

 receive it. 



It is shown, by writers on hydraulics, that this machine cannot 

 raise water wh.-n the angle which a line drawn centrally on th. 

 bends makes with planes parallel to the base of the cylinder is gi 

 than the angle which the latter makes with the hori/.on ; .uni 

 recommended that, in practice, the angle which the axis of the' cylinder 

 makes with the horizon should be between 40 and 00 degrees. Such u 

 machine is particularly useful when the water is mixed with i 

 weeds, and the like, which would spoil the action of a common pump. 

 For computations concerning the force requisite to turn the madam-, 

 and the quantity of water which it will raise in a given time, see 

 Gregory's ' Mechanics,' vol. ii. 



A machine consisting of a pipe wound spirally about the surface of 

 a cylinder, or cone, which is made to revolve about its axis when th.- 

 Utter is in a horizontal position, is called a tpiral-pump. At one 

 extremity of the spiral, water and air in nearly equal quantities being 

 allowed to enter, the former will, in consequence of the n-voluti. 

 forced up an ascending pipe which may be attached at the oUier 

 extremity. 



The Archimedian screw has been occasionally employed in modern 

 times to raise water from docks, basins, &c. ; and it might be used to 

 raise globular bodies, as cannon balls, from one level to a high 

 from the hold of a ship to the top of a wharf. A similar machine, 

 having the spiral detached from and revolving within the cylinder 

 which is about it, is also used. It is said to rajse more water than tin- 

 usual screw, but it cannot be elevated at a greater angle with tin- 

 horizon than 30, and its action is more easily impeded by the Bond or 

 gravel which is frequently mixed with the water. 



For the application of the screw to navigation, we must refer to 

 STEAM NAVIGATION. 



SCREW-JACK, a portable machine for raising great weights by the 

 agency of a screw. Portable jacks, which are sometimes worked by a 

 rack and pinion instead of a screw, are used for raising heavy carriages ; 

 the head of the jack being placed under the carriage, and raised by 

 turning the screw with a lever. The " Universal Screw-Jack " is a 

 great improvement on the common machine, as it all<>\. m well 



as vertical motion. In it the nut in which the vertical screw works 

 is fixed in a carriage resting on the framework that forms the base 

 of the machine, and capable of being moved upon it by means of 

 an horizontal screw turned by a ratchet lever. This kind of jack 

 is particularly useful on railways, where it affords a simple means of 

 lifting a carriage or engine that may have run off the rails, and then 

 moving it laterally until the wheels are in their proper position over 

 the rails. 



SCREW MANUFACTURE. The blanks for ordinary screws are 

 sometimes made of round rolled iron, cut into the required lengths, 

 and pinched when red-hot between a pair of dies in the chaps of a vice 

 while the heads are formed with a hammer, or the stamp of a fly-press! 

 Another plan is to form the blanks of iron wire, cut by a machine, and 

 have the heads struck up in a die without the application of heat. 

 After forming the head, the next process is filing or turning the necks 

 and heads in a lathe ; after which the nick, or groove to receive the 

 end of the screwdriver, is cut with a circular saw. The cutting of the 

 worm is sometimes performed in a lathe, thu blank being fixed in a 

 chuck, and projected during ite revolution between a pair of stationary 

 cutters; the longitudinal motion of the blank, and the inclination of 

 the thread, being determined by a regulating or pattern screw attached 

 to the mandril. Small screws are frequently wormed by a similar 

 apparatus turned by a winch-handle attached to the mandril; and 

 sometimes by means of asteel tap-plate. In another plan the worm is 

 formed by means of a pair of stationary cutting dies ; between which 

 the blank is projected by an apparatus which gives it an alternating 

 rotatory motion. The dies themselves regulate the size of the thread, 

 without the vise of a pattern screw, and they must therefore be changed' 

 for every variety of screw. The best screws are made to taper slightly 

 from the head downwards. 



Several attempts have been made to produce screws by casting. In 

 the ordinary method, the chief obstacle in the way of casting M 

 consists in the difficulty of removing the pattern from the mould. 

 Mr. Maullin has devised a method of overcoming this difficulty by an 

 apparatus for icrciring the patterns (of which a great number might be. 



