1478

THEORY AND PRACTICE OF ENGINEERING.

Book II.

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The quantity of Water-way to he given may lie determined by the condition of otherbridges, both above or below the situation of that to be constructed, taking care to makethe measurement at the time of the highest and lowest state of the river, and to observethe force and rapidity with which it moves.

The Number and Span of the Arches must depend upon these circumstances, as well asupon the nature of the foundations, the height of the banks, and the material to be em-ployed in their construction.

The greater portion of the following observations on the construction of bridges isselected from the “ Traite de la Construction des Pons, par M. Gautheyf as edited byINI Xaviers, a work deservedly celebrated throughout Europe for its research and practicalutility.

To estimate the Quantity of Water to which the Bridge must allow a Passage. — This isnot the same at all seasons; not only is it generally less considerable in summer than inwinter, but all rivers are subject to temporary increase, produced by abundant rains, orthe thawing of snow or ice; and the span of a bridge should be wide enough to allow themean quantity which the bed contains to flow through, and also afford room for the super-abundant quantity arising from floods: it will be found that this quantity of water bearssome proportion to the surface of the land which is drained off at that point of thecourse of the river where the bridge is to be erected; and it is very important to observecarefully the time which the superabundant quantity of water takes to flow through, orits velocity.

Gauthey's Instrument for measuring the Velocity of running Water, consists of a pallet 6*3inches wide and 3T inches high, made of tin plates ; a rod 3 feet 3-3 inches long is attached,flattened at its lower extremity, as well asat its upper, but in opposite directions, andhaving a round hole, £ of an inch in diameter,through its centre: two holes are bored inthe upper part, one oblong, the other circular,

■jlj of an inch above the first; the whole is soconstructed that the rod is in equilibrio aboutthe centre hole. The second part of this in-strument is an iron handle about 2 feet 3 incheslong, whose lower end is split and pierced so asto clasp the rod of the pallet about the middle,nearly in the same manner as the beam of abalance, by means of a gudgeon turning freely:to the upper extremity of this handle an arc ofa circle is attached, having the lower gudgeonfor its centre, also a brass, wire, likewise curvedin the arc of a circle. These arcs should beplaced opposite the holes in the extremity ofthe pallet-rod, and pass freely therein: thisflat arc is graduated, to show the differentdegrees of pressure acting on the pallet; toretain this latter, a spring is secured to thelower end of the handle, and it is attached tothe pallet-rod a little below the hole of thearc, by means of a movable ring in the thirdhole of the rod. To graduate the arc a tableof impacts compared to velocities must beformed, in which the first column indicates thecurrent's velocity per second, and the otherthe impact produced by this velocity. Thepallet-rod is then placed horizontally, the handlebeing firmly fixed; a light bag is fixed to thecentre of the pallet, and all the weights shownin the table are successively placed in it, and thespots on the arc where the pallet-rod becomesstationary is carefully marked and engraved.

To use this instrument a small piece of woodis placed in the arc of brass ; the upper extremityof the handle is held by one hand, and the lower part of the rod by the other: the palletis then placed in a current of river water, and the rod allowed to leave the hand bydegrees, inclining the handle, so that the pallet-rod is always vertical. The current byits pressure compresses the spring, and advances the small piece of cloth ; it is then takenout of the water, and, by pressing the rod till it touches the cloth, the degree to which thespring was compressed may be read off, and hence the velocity of the current: this instru-

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Fig. 2609. Fig. 2700.

Fig. 2701.