446 



SCIENCE OF AGRICULTURE. 



Part II. 



265^. Tfie construction of a gate best adapted for opening and shvtting, is next to be 

 considered. All gates, after being hung, have from their gravitation a tendency to deviate 

 from tlieir original position, to sink at the head or falling post, and thus no longer to open 

 and shut freely. If the construction and hanging of the gate were perfect, this could 

 not possibly take place ; but as the least degree of laxity in trussing the gate, or want 

 of firmness in fixing the post in the ground, will occasion, after frequent use, a sensible 

 depression at the head, it becomes requisite either to guard against it as much as possible, 

 in the first construction ; or to have a provision in the design of the upper hinge (Jig. 

 397.), for rectifying tlie deviations as they take place. 



2853. In order to understand the construction best calculated to resist dejrressiov, sup- 

 pose a gate hung, and resting on its heel (Jig. 396 O, acting as a strut, and maintained 

 there by its upper liinge (rf), acting as a tie, then the bottom rail of the gate considered as 

 representing the whole, becomes a lever of th6 second kind, in which the prop is at one 

 end (c), the power at the other (g), and the weight placed between them in the line of the 

 centre of gravity of the gate (t). Now, as two equal forces, to hold each other in equili- 

 brium, must act in the same direction, it follows that the power acting at the end of the 

 lever (g), will have most influence when exerted at right angles to it in the line (g, e), 

 but as this cannot be accomplished in a gate where the power must be applied obliquely, 

 it follows, that a large angle becomes requisite ; that the greater the angle, the greater the 

 power, or in other words, the less the strain on the construction of the gate, or the less the 

 tendency to sink at the head. The half of the right angle [c,e,g] seems a reasonable 

 limit, by which, if the power requisite to hold the weight in equilibrium, when acting at 

 a right angle, be as the side of a square of the length of the lower bar of the gate (^', c), 

 then the power requisite to effect the same end, when acting at an angle of 45 degrees, 

 is as the diagonal to this square {g,h". By changing the square to a parallelogram, 

 the relative proportions will still be the same, and the advantages and disadvantages will 

 be rendered more obvious. 



2854. Parker s compensation hinge for gates which are much in use (Jig. 397.), is an 



excellent corrective to their falling ; all that is necessary when the gate sinks at the head 

 is to screw it up by tlie nut (a), till it regains its original position. For road and farm 

 yard gates the hinges are valuable parts of the construction. 



2855. A gate should be so hung as to haw two Jails ; one to the hanging post, to make it 

 catch, and the other to a point at a right angle with the gateway, so as to keep it fully 

 open. To effect this purpose, having set the post perpendicular, let a plumb-line be 

 drawn upon it : on this line, at a proper height, place the hook, so that it may project 

 three inches and a half from the face of the post; and at a convenient distance below this 

 place the lower hook an inch and a half to one side of the perpendicular line, and pro- 

 jecting two inches from the face of the post ; then place the top loop or eye two inches 

 from the face of the hanging style, and the bottom loop three inches and a half: thus 

 hung, the gate will have a tendency to shut in every position. A gate so hung will have 

 a tendency to shut in every position ; because if the weight of the gate be represented by 

 a diagonal line from the heel to the head, this, by the resolution of forces, is resolvable 

 into other two lines ; one perpendicular, and the other horizontal ; the former repre- 

 senting that part of the weight which presses in a perpendicular position, and the latter 

 that part of the weight which presses in a horizontal direction, and gives the gate a 

 tendency to shut. {Northu7nh. Rep. 63.) 



2856. Gates are generally constructed of timber, and whatever kind may be used, it is 

 essential that it be well seasoned, as without attention in this respect, they are soon de- 

 ranged in their structure by the heat of the sun : they should also be well and correctly 

 put together. Oak is undoubtedly the best sort of wood for the purpose, where dura- 

 bility is the object ; though some of the lighter kind of woods, as deal, willow, &c. will 

 often last a great length of time, as, from their lightness, they are not so apt to destroy 

 themselves. The lighter gates are made towards the head or opening part the better, 



