Book III. 



USEFUL DECORATIVE BUILDINGS. 



353 



or of roofs, consists in the judicious composition of ties and struts ; the former always re- 

 sisting a drawing or twisting power, and the latter one of a pressing or crushing nature. 



321 

 319 



a_ 



a 



h 



322 



1795. By the maintenance of a gate* s position, we mean the resistance to that tendency 

 winch most gates have 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 de- 

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

 the first construction ; or, to have, as in N. Parker's gate, a provision in the design of the 

 upper hinge, for rectifying the deviations as they take place. In order to understand the 

 construction best calculated to resist depression, suppose a gate hung, and resting on its heel 

 (Jig. 322. c), acting as a strut, and maintained 

 there by its upper hinge ((/), 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 (i). Now, as two equal 

 forces, to hold each other in equilibrium, must 

 act in the same line of 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 or parallel to the line of 

 gravity (g e) ; but as this cannot be accom- 

 plished in a gate where the power must be 

 applied obliquely, it follows, that a larger 

 power becomes requisite ; but that the less 

 the obliquity, the less will be the power, or in other words the less the strain on the con- 

 struction of the gate, or the less the tendency to sink at the head. The half of the right 

 angle (g e c), 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 relative proportions will still be the same, 

 and the advantages and disadvantages will be rendered more obvious. (For g d is not 

 to d c, as g h is to h c.) It is evident from this principle, that gates whose upper line is 

 concave, or falls from the posts or piers to the centre (Jig. 320.), are more fitting, and 

 consequently more beautiful, than such as are of an opposite description (Jig. 321.) 

 But a person totally ignorant of mechanical principles, but of good taste in visual mat- 

 ters in general, might prefer the latter, which shows, that a just or true taste must be 

 founded on science or reason, and is by no means so vague and indefinite, or arbitrary 

 an exertion of judgment as many are apt to imagine. 



1796. Compensation-hinges. Where there is no choice between a constructiori calcu- 

 lated to resist sinking, and the common form, then the corrective or comj>e?isation-hinge 

 of N. Parker (Jig. 323.) is very proper for division-gates in parks or drives ; but a sci- 

 entific construction, either polished or rustic (Jig. 326.), maybe easily contrived for 

 gates in forests and farms. When Parker's hinge is used, all that is necessary, when the 

 gate sinks at the head, is to screw it up by the nut (a, fig. 323.) till it is replaced in its 

 original position. 



1797. With respect to facilitatbig the motioji of gates, that is to be done by lessening the 

 friction of their hinges. Friction is as the extent of rubbing surface, and the weight ; 

 therefore, of the two hinges of a gate, the friction of the heel, when a pivot, is by much 

 the least, as the rubbing there is limited to one point, instead of the whole surfaces of two 

 cylinders. Whatever, therefore, has a tendency to throw the preponderance of weight 

 on the heel, must lessen the friction of the upper hinge. This will be accomplished in 



A a 



