492 



Book IV. G AXES APPROPRIATE TO AGRICULTURE. 49 



a gate are all ties ; the diagonal and perpendicular ones strutts. On the judicious 

 combination of these ties and strutts depends the absolute strength of the gate ; and 

 on their lightness, and on the general form of the gate, depends its adaptation for 

 opening and shutting by means of hinges. 



3077. The construction of a gate best adapted for opening and shutting is next to be 

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

 deviate from their 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 truss- 

 ing 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 {fig. 493.) for rectifying the deviations as thev take 

 place. 



3078. hi order to understand the construction best calculated to resist depression, suppose a gate hung, 

 and resting on its heel {fig. 492.c) acting as a strutt, and maintained thereby its upper hinge (rf) acting as 



a tie, then the bottom rail of the gate considered as 

 representing the whole, becomes a lever of the 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 (?). Now, 

 as two equal forces, to hold each other in equilibrium, 

 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 {g 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 rela- 

 tive proportions will still be the same, and the advantages and disadvantages will be rendered more 

 obvious. 



S079. Waistell and Parker have paid great attention to the construction of gates for many years. 

 More than fifty years ago, Waistell circulated among his friends plans for ornamental gates with semi- 

 oval and semicircular braces, and such gates {fig. 496.) have now become general. Parker has directed 

 his principal attention to the hangings and fastenings of gates ; and his forms of latches, hinges, itc, es 

 well as his turnpike-gates {fig. 495.), are also very general. 

 3080. The construction of the gate is thus given by "Waistell, The head {fig. 493. a) and heel (6) are to 



493 



-,d 



be fonned of oak, and the bars and braces of foreign fir. " If inferior materials are used, they may be 

 made a little thicker, but the breadth should remain the same. 



The heel of the gate to be about 



The head of ditto 



The top rail, or bar, vertical piece 



Ditto horizontal piece 



The bottom bar ---.... . 



. The other four bars, and the four braces - - - -2 - ' 



The dimensions in column under A are taken on the face of the gate ; those in the column under B are 



taken in the direction of its thickness. Narrow and thick bars, when braced as in this design, are stronger 



than broad and thin ones, containing the same quantity of timber, and they also oppose a less surface to the 



Kk 2 



3i - 



n - 



3i - 

 2| 



