Page:The New International Encyclopædia 1st ed. v. 17.djvu/786

SCKANTON. 712 SCREW. the Di'lnwart', Lackawami:! ami WostiTii Kailway, ana siiv.'d for many viais as its jviosident, being niso picsiilcnt of otbcr railroads and transporta- tion louipanics. SCREAMER. Any of tlirec curious South Anieritan birds, the relationship of which has been a matter of eonsiderable discussion. They are now regarded as most nearly related to the anseriform birds and forming a fiimily (Palame- deida?). The bill is rather short, conical, curved at the extremity ; there is a bare space around the eyes; the toes are long: each wing is fur- nished with two strong spurs, one at the bend of the wing and a smaller one nearer the tip. The horneiT screamer, or 'anhima,' 'cliaha,' or 'kamichi' {Pnlamcdca coniiitu). inhabits swamps in Brazil, (iuiana, and .Argentina, and feeds on the leaves and seeds of aquatic plants. It is of a blackish-brown color, is nearly as large as a turkey, and has somewhat the appearance of CRESTED SCREAMER {Cbauna cristata). a gallinaceous bird. It receives its name from its loud and harsh cry. From the head, a little behind the bill, there rises a long, slender, mov- able horn, the use of which is not clear. The spurs of the wings are supposed to be useful in defense against snakes and other enemies. Closely allied to this is the genus Chauna, to which belongs the chauna, or crested screamer {Chauna crisiata) , a native of Brazil, Para- guay, and Argentina, the head of which has no horn, but is adorned with erectile feathers. The plumage is mostly lead-colored and blackish. The wings are armed with spurs. It is capable of domestication, and is sometimes reared with flocks of geese and turkeys, to defend tliem 'from vultures, being a bold and powerful bird. Con- sult: Evans, Birds (London, 1900) ; Sclater and Hudson, Aryentine OrnUhology (London, 1888). SCREECH OWL. See Owl. SCREEN (OF. escren, escrein, escran, Fr. vcran, screen, probably from OHG. scrannn, Ger. Schranne. bench, shambles, railing, grate, court). In architecture, an inclosure or partition of wood, stone, or metal work. It is of frequent use in dnirches, where it shuts off chapels from the nave, separates the nave from the choir, and frequently incloses the choir all round. The rood-screen is that on which most labor is usually bestowed. In Kngland many beautifully carved screens iu stone, enriched with pinnacles, niches, statues, etc., remain, such as those of York, Lincoln, and Durham ; and specimens in wood, carved and painted, are common in parish churclies. The term 'screen of columns' is also applied to an open detached colonnade. SCREW (OF. escroue, cscroe, escro, Fr. ecrou, screw, perhaps from Lat. scrohs, ditch, trench, or more probably from Lat. scrofa, sow). An in- clined plane wrapped around a cylinder in such a manner that the height of the plane is parallel to the axis of the cylinder. If the screw is formed upon the inner surface of a hollow cylin- der it is usually called a nut. Defined less tech- nically a screw is a solid cylinder having a heli- coidal rib, ridge, or thread projecting from its surface. Historically the invention of the screw is ascribed to Archimedes (B.C. 250). It was used by the Romans of the Empire in their wine and oil presses and was probably familiar to most of the Mcditerianean peoples at the beginning of the Christian Era. It is one of the most extensively used of the elementary mechanisms and is employed in the manufacture and operation of nearly all struc- tures, machines, and mechanisms. The force for operating the screw is universally applied at the end of a lever arm at right angles to the axis of the screw. When used for transmitting energy the screw is generally operated in connection with a nut; either the screw or the nut may be fixed, the other lieing movable. Until the nineteenth century the manufacture of screws was a rather crude process of forging and cutting with hand tools. At present large screws for transmitting energy are made on screw-cutting lathes, the cylinder of metal being rotated by the lathe in front of a tool, which advances at a uniform speed parallel to the axis of the work and thus cuts a helicoidal groove. Generally such screws are made with rectangular threads. The most common forms of screws are wood screws for cabinet and car- penter 'ork and machine screws for metal work, ilachine screws are made with care to secure precision in the forms and dimensions of the thread, but wood screws are more roughly made. These small screws were little known or used before 18.36, being rudely made by hand with imperfect tools. The head was forged or swaged by a blacksmith ; the thread and nick were formed by the use of hand dies and hack saws. In 1836, as a result of an American invention, the old hand tools were transformed into machines having the capacity of imparting to each tool its proper motion. The swaging hammer became the heading machine, receiving the end of a coil of wire and regularly cutting the required length for a blank, which then received such a blow as to 'set up' one end of the wire to form the head — ■ the operation continuing automatically until the whole coil was made into blanks. These blanks were then handled individually and presented to organized machines, first for shaving the head, then for nicking, and lastly for cutting the thread. The above constitutes the second era in this manufacture: and such machinery, partly automatic, was all that was in use before 1846. Then a third era ensued, and an entire revolution was effected by constituting the machines en- tirely automatic. The blanks arc by this system supplied in mass by the operator, the machine