RILLS, KIVULETS, AND CASCADES. 



715 



double set of cross bearers, by means of 

 which an additional set of principal beams 

 is introduced, adding considerably to the 

 length of the bridge. A space of 40 feet 

 may readily be spanned by this means, 

 by making the beams resting in the abut- 

 ments cover a space each of 15 feet, and 

 the level pieces in the centre 10 feet. A 

 very simple hand-rail may be constructed, 

 consisting of forked uprights, having a 

 strong rope, or withy plaited of flexible 

 heath, running through the forks. 



Fig. 1000 is a bridge of simple con- 

 trivance, and suited for spaces not exceed- 



Fig. 1000. 



ing 15 or 18 feet. It is slightly curved, 

 and, after previous remarks, requires no 

 explanation. Such bridges are not only 

 useful, in so far as they serve for the 

 purpose of communication, but they have 

 value as objects enriching the landscape, 

 more especially when seen from the walks 

 which may be at a lower level. It is im- 

 proper to cover the roof of such bridges 

 with gravel, as it is both out of character, 

 and it lays an unnecessary load upon 

 them, which their construction is not 

 suited to carry. 



§ 5. — RILLS, RIVULETS, AND CASCADES. 



In this style, artificial rills, rivulets, 

 and cascades, may be introduced : if they 

 exist naturally, even to a limited extent, 

 they may be extended; or if the situation 

 is naturally favourable to their introduc- 

 tion artificially, happy effects may be 

 produced ; but they must be natural to 

 the scene — and such scenes, says the 

 author of "Observations on Modern 

 Gardening," "commonly require every 

 accompaniment which can be procured 

 for them. Mere rocks, unless they are 

 peculiarly adapted to certain impressions, 

 may surprise, but can hardly please. 

 They are too far removed from common 

 life — too barren and inhospitable — rather 

 desolate than solitary, and more horrid 

 than terrible ; so austere a character 



cannot be long engaging, if its rigour be 

 not softened by circumstances, which 

 may belong either to those or more 

 cultivated spots ; and when the dreari- 

 ness is extreme, little streams and water- 

 falls are of themselves insufficient for the 

 purpose — an intermixture of vegetation is 

 also necessary ; " and, we may add, the 

 softening hand of art to a limited extent. 

 " If such a scene," says the same autho- 

 rity, " occurs within the precincts of a 

 park or garden, no expense should be 

 spared to meliorate the soil. Without 

 some vegetation amongst the rocks, they 

 are only an object of curiosity, or a sub- 

 ject of wonder; but verdure alone will 

 give some relief to the scene ; and shrubs 

 and bushes, without trees, are a sufficiency 

 of wood. The thickets may also be extend- 

 ed by creeping plants, such as pyracan- 

 tha, cotoneasters, and ivy, to wind up the 

 sides, or cluster on the top of the rocks." 

 In such situations artificial rills, rivulets, 

 and cascades, may be appropriately imi- 

 tated; and their construction requires 

 much taste, as well as a knowledge of 

 the resisting power of fluids, in forming 

 the dam or bank at the end of the reser- 

 voir. " With respect to strength," is as 

 Mr Loudon, " the pressure of water says 

 its depth ; and consequently a B dam, whose 

 section is a right-angled triangle, and 

 whose hypothenuse forms an angle of 45° 

 with the base formed of any material of 

 greater specific gravity than water, would, 

 so far as strength is concerned, hold in 

 equilibrium a body of still water of a 

 depth equal to its perpendicular. If the 

 hypothenuse, or sloping side, be placed 

 next the water, it will more than hold 

 the water in equilibrium, by the weight 

 of the triangle of the water superincum- 

 bent on the dam or bank. 



" That the materials of the bank must 

 be of a nature impervious to the water, 

 and also must adhere to the base or bot- 

 tom, so as not to admit water to escape 

 beneath it, are obvious conditions of the 

 foregoing proposition. The practice of 

 forming dams or heads is derived from 

 this theory; but to guard against acci- 

 dent, the base of the triangle is always 

 more than three or more times greater 

 than its height. The slope next the 

 stream may form an angle with the hori- 

 zon of from 40° to 20°, and that on the 

 lower side is regulated by the uses of the 



