GATES.

3G3

1354. Section Ninth —Gates —The reader, desirous to discuss the principleson which gates ought to he constructed, is referred to the Book of Farm Imple-ments and Machines , where he will find the subject fully entered into. Wejiroceed at once, therefore, to illustrate examples for practice.

1355. Fig. 572 represents field-gate with diagonal strut, without the upfil-lings, where a c is the heel-post, h dthe head-post, and c b and a d the topand bottom rails of the rectangularframe of a field-gate ; and a b is thediagonal, which converts the rectanglea c b d into two triangles a c b andb a d.

1356. In field-gates constituted en-tirely of wood, the diagonal should in-variably be applied as a strut, to risefrom the foot of the heel at a, and terminate at the top of the head-post at b.Placed in this position, the diagonal a b supports the swinging end of the gateb d, by its resistance to compression ; which it is well adapted to perform by thearea of its cross section being considerable, and hence capable of resistinglateral flexure.

1357. But a field-gate is liable from various causes to be forced up at thehead-post b d, however well the diagonal is adapted to prevent the upper railfrom being depressed by any undue weight exerted upon its end at b. Theadvantages of a tie are the converse of a strut. If a tie, therefore, is placedfrom c to the opposite angle d, crossing the strut a b in its centre—and an ironbar makes a perfect tie, the cohesion of which is such that a very small sec-tional area is sufficient for the purpose—the two antagonistic forces of thewooden strut and the iron tie, acting each in its own sphere, preserve withinthe whole structure the most perfect equilibrium.

1358. Defective Field-Gate. —A very common form of field-gate to be seen inthis country is shown in fig. 573, and applying correct principles to it, weshall find it defective in several mostessential particulars. It has a strut a b,but instead of extending across the en-tire diagonal to c, it stops short at thecentre of the gate at b. The part ofthe top b c is liable to be broken off byany undue force being exerted upon itat c when it is converted into a lever,whose fulcrum is supported at b by theend of the strut a b. It has also a tieb d, which is not only made of a wooden rail, but it does not extend across therectangle to e, and in no part does it cross the strut a b, so as to act with it inmaintaining an equilibrium of forces. The consequence in practice is, thatthis form of gate is very frequently fractured at the head-post c d, and falls tothe ground at d.

1359. Trussed Gate. —-The principle of trussing has been successfully intro-duced into the wooden field-gate by Sir John Orde of Kilmory in Argyllshire.Fig. 574 shows the rectangular form trussed, so as to make a compact firmstructure. The heel-post a b, the head-post c d, and the upper and lower railsa c and b d, form the rectangular frame. The truss consists of four bars ofwood, a e, b e, c e, and d e, each of which abuts into an angle of the rectangle,

Fig. 573.

REOTANGOl.AR GATS WITH E (AGONAL STRUT.