SCR 



SCR 



It may be remarked, that thefe forts of fcreen-fences ftiould 

 be more attended to by the proprietors of land in all expofed 

 fituations, as they have much efFeft in promoting the im- 

 provement of different forts of live-ilock in fuch expolures. 



ScR&ES-Plantations, and Planting, a term fometimcs ap- 

 plied to fuch belts, clumps, or hedges, as are planted with 

 a view of affording (helter in expofed fituations. And Mr. 

 Marfhall, in his work on " Landed Property," confiders 

 the effeft of fuch plantations, not merely that of giving 

 ihelter to the animals lodging immediately beneath them ; 

 but likewife in breaking the uniform current of the wind ; 

 (hattering the cutting blafts, and throwing them into eddies ; 

 thus meliorating the air to fome diitance from them. And, 

 moreover, he is of opinion, that living trees communicate 

 a degree of aftual warmth to the air which envelopes them. 

 It is at leaft, he thinks, a probable truth, that where there 

 is life there is warmth, not only in animal but in vegetable 

 nature. The feverelt froit rarely affefts the fap of trees ; 

 it confequently appears to him, that trees and fhrubs, pro- 

 perly difpofed in a bleak fituation, tend to improve the 

 lands fo fituated in a threefold way for the purpofes of 

 agriculture : namely, by giving Ihelter to itock, by break- 

 ing the currents of winds, and by communicating a degree 

 of warmth or foftnefs to the air in calmer weather, or when 

 the feafons are more mild. 



Beiides, it is faggefted that the proper difpofal of fcreens 

 in this intention, is in lines acrofs the moft offenfive winds, 

 and in fituations bell calculated to break their force. Placed 

 acrofs vallies, dips, or more open plains in bleak expofures, 

 they may, he fuppofes, be of Angular ufe ; and alfo on the 

 ridges, as well as on the points and hangs of hills, and other 

 elevated places. 



And in regard to the width of fuch fcreens, they ought 

 generally, he thinks, to be regulated by the value of the land 

 for agricultural ufes, and the advantages of the fituation for 

 the fale and delivery of timber. In ordinary cafes, it is con- 

 ceived that from two to four ftatute poles may mollly be 

 confidered as an eligible width for fuch purpofes. With 

 refpe£t to the nature or fpecies of plants which are molt 

 fuitable, they muft conftantly be adapted to the given foil 

 and fituation. Mr. Marfhall thinks, that in bleak and barren 

 fituations the larch will generally be found the moft pro- 

 fitable as timber. But being deciduous, it does not in 

 winter afford fo much flielter as the common fir. A fcreen 

 to Ihelter live-llock fhould be clofe at the bottom : it is 

 otherwife injurious rather than beneficial. Not only the 

 blaft acquires additional current, but fnow is liable to be 

 blown through, and to be lodged in drifts on the leeward 

 fide, to the annoyance and danger of (luep that have re- 

 paired to it for fhelter. A larch plantation margined with 

 firs, and thefe headed at twelve or fifteen feet high, would, 

 he fuppofes, afford the required fhelter for a length of years. 

 Tiie firs, thus treated, would be induced to throw out 

 lateral boughs and feather to the ground ; while the larches, 

 in their more advanced ilate of growth, would, by permit- 

 ting the winter's winds to pafs through the upper part of 

 the fcreen, break the current and mellow the blaft, and in 

 this way a complete (helter be provided ; but in other more 

 genial fiuiatronB. the beech, by retaining its leaves in winter, 

 «fpecially while it is young, forms a valuable fcreen. And 

 that, if the outer margins were kept in a ftate of coppice- 

 wood, and cut alternately, and the middle ranks fuffen d to 

 rife as timber-trces, the triple purpofe of fcreen-plantations 

 ■night, he imagines, be had in an eminent degree, and ainioll 

 in perpetuity, with but little trouble. And in deep-foiled, 

 talc diflrifts, which not unfrcquently want fhelter, fcreens of 

 •ak might, he conceives, be managed in a Cmxlar way : and 



hollies, or other hardy evergreen^^, planted as underwood in 

 groves of either of the above defcriptions, would, if fuitable 

 fituations were afTigned to them, he fuppofes, afiiil much in 

 this intention, efpecially where proper care was taken in 

 planting and protecling tliem. 



I'his fort of plantation, when firft formed, fhould always 

 be well fuited to the nature of the expolure, both in refpeft 

 to the kinds of the trees and plants which are made ufe of 

 for the purpofe, and the fpace or extent of the planting 

 which will be neceflary for effefting tlie bufincfs. In com- 

 mon, narrow flipa never anfwer the intention in any perfect 

 manner, as the trees neither rife well, nor afford the fhelter 

 which IS wanted in fuch cafes. See Shelter. 



ScKEK}i-Bulihead, in Ship- Building, the after-bulkhead 

 under the round-lioufe, when the fhip has a walk, or balcony. 



SCREW, or Scrub, Cochlea, in Mechanics, one of the 

 fix mechanical powers ; chiefly ufed in prefling or fqueezing 

 bodies clofe, though fometimes alfo in raifiug weights. See 

 Mechanical Powers. 



The fcrew is a right cylinder, as A B (P/a/^ XXXVIII. 

 Mechanics, Jig. I . ) furrowed fpiral-wife ; it is generated by the 

 equable motion of a right line F G (Jig. 2.) around the fur- 

 face of a cylinder ; while, at the fame time, the point I de- 

 fcends equably from F towards G. Or, it may be conceived 

 to be made by cutting a piece of paper into the form of an 

 inclined plane, or half-wedge, and then coiling it round a 

 cylinder ; fo that its adtion depends on the fame principle* 

 as that of an inclined plane. Tiie force tending to turn the 

 fcrew round its axis may be confidered as applied horizon* 

 tally to the bale of the wedge, and the weight which is to be 

 raifed as acting vertically on its inclined furface : the cir- 

 cumference of the cylinder will reprefent the horizontal 

 length of the wedge ; and the difla.icc between the thread*, 

 meafured in the direftion of the axis, w ill be its height, pro- 

 vided that the threads be fingle ; confequently, the force* 

 required for the equilibrium are to each other, as the height 

 of one Ipire to the circumference of the fcrew. But befide* 

 thefe forces, it is neceflary that fome obftacle be prefent, 

 which may prevent the body, on wliicli the fcrew atls, from 

 following it in its motion round its axis ; otherwife there can 

 be no equilibrium. If the furrowtd fuiface be convex, the 

 fcrew is faid to be male; if concave, it \s Jemale. 



Where motion is to be generated, the male and female 

 fcrew are always joined ; that is, whenever the fcrew is to 

 be ufid as a fimple engii'e, or mechanical power ; and when 

 thus fitted togetlur, thty are fometimes called a fcrew and a 

 nut. The nut afts on the fcrew with the fame mechanical 

 power as a linglc point would do, fince it only divides the 

 preflure among the different parts of the fpire. When joined 

 with an axis in peritrochio, there is no oecafion for a fe- 

 male ; but in that cafe it becomes part of a compound 

 engine. 



The fcrew cannot properly be called a fimple machine, be- 

 caufe it is never ufid without the application of a lever, or 

 winch, to affifl in turning it. Sometimes the fpires of a 

 fcrew are made to aft on the teeth of a wheel, when a very 

 flow motion of the wheel, or a very rapid motion of the 

 fcrew, is required for the purpofes of the machine. 



Screw, Dctlrine of the. i. If, as the compafs, dcfcribfd 

 by the power in one turn of the fcrew, is to ilic interval or 

 diftance between any two immediate threads, or fpiral wind- 

 in-.^s, as B I (meafured according to the length of the fcr»w), 

 fo is the weight or refiltance to the power ; then the power 

 and the refiflancc will be equivalent one to the other ; and, 

 confequently, the power being increafed, fo as to counteract 

 the friftion of the fcrew, wMiich is very confiderable, will 

 overcome the refiltance. For it 14 etidcut, that m ooo turn 



