260 



8CH I w 



SCREW IMNK 



In Catholic elmrches it may liave an altar at each 

 side, iiml is fni|iicntly a structure of winic si/c, 

 with n staircase leading to the rood-loft on the top, 

 where the cross or crucr conspicuous at 



the entrance to the choir. The loft in also used 

 fur certain religious functions and ceremonies. In 

 Kngland inanv beautifully carved screens in stone, 

 enriched wiili pinnacles, niches, statues, &c., 

 remain, such ax those of York, Lincoln, !)iirlutin, 

 itc, ; and specimens in wood, carved and (tainted, 

 are common in parish churches. that at Hnrliorton, 

 near Totnes, which U represented in our illustra- 

 tion, iN'ing one of the finest in the kingdom. In 

 France the screen round the choir is sometimes 

 the subject of beautiful sculptures, as at Amiens 

 and Paris. In halls of castles nnd mansions 

 there was usually a wooden screen at one end to 

 separate the entrance-door and a passage from the 

 hall. Over this was a gallery. The term 'screen 

 of columns' is also applied* to an open detached 

 colonnade. 



Screw. Under ROTATION it is pointed ont 

 that tbe most general displacement which a rigid 

 body can experience may be represented bv a 

 crew motion ul>oiit ami along a definite axis. The 

 kinematic characteristic of screw motion is easily 

 studied by help of any ordinary screw working in 

 its nut We may tix the nut and consider the 

 motion of the screw ; or fix the screw and consider 

 the motion of the nut. In either case we find a 

 certain translation associated with a proportional 

 amount of rotation. The ratio of the translation 

 t the associated rotation is called the/<>V>7i of the 

 screw. In an ordinary single-threaded screw the 

 practical measure of the pitch is the distance 

 net ween the threads, and is simply the translation 

 for one complete rotation. In dynamics it is more 

 scientific to take the pitch as the translation for a 

 rotation through the radian or unit angle (see 

 CIRCLE). The ratio of the scientific to the 

 practical pitch is the snnte as the ratio of the 

 circumference of a circle to its radius. The 

 smaller the pitch, the lexs is the translator^ 

 motion for a given rotation ; nnd when the pitch is 

 made zero, the screw motion is reduced to a pure 

 rotation. On the other hand, the larger the pitch, 

 the greater is the translation for a given rotation ; 

 M that a screw of infinite pitch corresponds to a 

 pure t initiation. Thus screw motion includes all 

 possible kinds of motion, translation and rotation 

 being limiting cases. 



l>\ nnmicalh , screw motion is said to be produced 

 by a irrmrh, which is the. most general quantity 

 nf the force type. When tin- screw motion is re- 

 duced to translation, the wrench becomes force in 

 the usual sense of the won! ; and couple, which 

 produce* lolation, is the other limiting case of 

 wrench. Tin- simplest conce|itioii of a wrench is 

 - from the comliination of pushing and 





twisting nccdi-d to 1*>re holes with a gimlet. 



Tin- screw i- a very important element in 

 machinery. In itself, however, it is not complete, 

 being only half of the .^ ,,/,,, m elementary 

 mechanism to which it lielongs. The complete 

 ni'-'-linnisin is well illustrated by the combination 

 of screw and nut. Ily making the pitch zero, we 

 pass to the one limiting ease known as the 



ftnir ; ami by making the iiitch infinite we get the 

 other limiting eae vi/. U>6 s/ir/ini/-iunr. These 

 three combinations form the simple' elements of all 



lliaehi 



One of the mot im|H>rtnnt uses of the screw (as 

 in the screw pics, , j, to apply or sustain a large 

 pressure in the direction of Its axis by means of a 

 eomparatirvly small couple acting about it nxis. 

 We shall suppose the couple to be applied bv a 

 force, /', acting at the end of a lever attached to 

 the screw; and that the result is a thrust, /. If 



a is the arm of the lever, and /) the pitch, the prin- 

 ciple of work (see ENKRCY) gives us the relation 

 Fa = Pp. Hence the smaller the pitch the greater 



the pressure exerted by the screw for a given couple 

 acting round ite axis. The pilch is diminished by 

 making the thread of the screw liner. Hut, as this 



also makes it weaker, we see that there must be 

 a practical limit to increasing the advantage of 

 the screw in this way. So far we have neglected 

 friction, which must, however, be considerable in 

 all screw -pail's because of the great pressures exist- 

 ing between the opposed faces of the threads of the 

 screw and nut. The power applied, f'<i, must, 

 therefore, be greater than the effective work done. 

 In some cases, when the pitch is small, the 

 efficiency, or ratio of useful work done to work 

 expended, may he reduced to one third. If the pitch 

 is very large the action of the screw may oe re- 

 versed, the driving force being the thnisi, /', nnd 

 the resistance the couple, Fa, opting the rotation 

 of the screw. In most cases the pitch is too small 

 for reversal, the couple brought into play by the 

 friction always being sufficient to prevent motion. 

 It is this non-reversibility that gives the screw ita 

 peculiar virtue in holding together parts of a frame 

 or machine. For the screw-propeller, see SHIP- 

 iu:iu>iN(>, page 404. 



Screw Bean. See MEZQI'ITE. 



Screw-nails, called in the trade ' wood screws,' 

 are made from mild steel and iron ; or from brass, 

 copper, and zinc, when others would l>e destroyed 

 by rust. Wood screws were in use long before the 

 year 1760, when the brothers Wyatt obtained a 

 patent for cutting screws bv machinery. Before 

 1817, wood screws, In-ing nearly all forged and then 

 shaped by hand, were very expenshc ; but at that 

 time a patent was granted to John Colbert, a 

 German watchmaker, who made the first automatic 

 machinery for this purpose. In the year 1854 a 

 greatly improved machine was introduced from 

 America, which may be said to have revolutionised 

 screw-making. These machines are complicated, 

 and expensive in first cost and also to keep in 

 operation. The wire is supplied to the heading- 

 machine, which cuts off a piece, and a blow from 

 a die stamps a head on it. It is then turned and 

 nicked, after which, in a worming-nmchinc, a 

 cutter passes several times along the blank and 

 forms the thread nnd gimlet point. One person 

 can attend to several of these machines. Consider- 

 able success 1ms Iteen attained in rolling the threads 

 on screw blanks. Two reciprocating plates, or 

 revolving discs, are made from hard steel, nnd 

 grooved at an angle ; the screw blank is rolled 

 between them under great pressure, receiving the 

 reverse impression of the grooves which form the 

 thread. The annual consumption of wire for 

 making wood screws in England amounts to 9000 

 tons. They are principally made in Hiiminglmm, 

 and one firm alone can produce 150,000 gross 

 weekly. 



Screw Pine ( fiim/nntit), a genus of plants of 

 the iwitural order Piindnnaceie, natives of the tropi- 

 cal parts of the east and of the South Sea Islands. 

 Many of them are remarkable for their adventitious 

 roots, with large cup like s/mniiinlfx, which their 

 branches send down to the ground, and which 

 serve as props. Their leaves are sword-slinped, 

 with spiny edges, and are spirally arranged in 

 three rows. In general appearance, when mi- 

 branched, they resemble gigantic plants of the 

 pine-apple, whence their popular name. /'. odorrt- 

 n'MMNtU is a widely-diffused species; a spreading 

 and branching tree' of 25 feet high, much used in 

 India for hedges. It grows readily in a poor soil, 

 and is one of the first plants to appear on newly- 

 formed islands in the Pacific. The male llowere 



