638

THEORY AND PRACTICE OF ENGINEERING.

Rook II.

these two tides make the circuit of Britain in twenty-eight hours, during which period theprimitive tide has made the circuit of the globe, and nearly 45° in addition.

High water on the Thames at Londonbridge takes place at the moment when itis low water at the mouth of the river, thesurface of the water at London being 40 feetabove its level at the German Ocean at thesame time.

Large rivers in which the tides occur donot have a regular descent of surface towardsthe sea, but a varied outline produced by con-tinual motion.

The Bore , which is an accumulation of waterin large rivers at the time of flood, arises fromthe narrowness of the outlet preventing theentire discharge of the water before thereturn of the next tide, which it meets as itflows in an opposite direction, and conse-quently causes an elevation of water aboveits natural level, and which is often extremelydangerous to small craft and to navigation.

The wave so formed rolls with great violenceup the river, pressed forwards by the accu-mulating force of the tide, until it is lost ordies away. In the river Severn the boreoften rises 10 feet, and in the Amazon it issaid to mount upwards of 100.

A variety of instruments are made use ofin our harbours to measure the tides; thatwhich is found to answer the best is formedof a pole, 24 feet in length, and 6 inches indiameter, supported upon legs firmly lashedtogether with ropes, ballasted with pig-ironlaid over the bottom. On the pole is placeda perpendicular graduated scale, made of dealor other wood, and the whole is kept steadyby several guy ropes attached to the top.

Measuring rods, made of ^ inch deal, witha cork float at the bottom, in the form of acube of 3 inches, work in boxes attached tothe pole: these rods, graduated in feet andinches, pass through staples which steadythem ; and as the water is admitted into theboxes through a hole in the bottom, the rodsrise with it, indicating to what height in agiven time the tide rises or falls.

Origin of Rivers .—The condensation ofvapour on the tops of the highest mountains isthe origin of some of the largest rivers, andthe source of all is dependent upon meteorolo-gical causes. That water which falls from theclouds and enters some depth into the earthagain issues forth as a spring, and, gainingstrength in its course by the addition of otherstreamlets, continues to flow on till it reachesthe ocean, after being sometimes diverted inits course by the various obstacles that areopposed to it. On tracing the descent of ariver, we perceive that the gradation frommasses of rock to grains of fine sand is almostimperceptible. At its commencement the water trickles drop by drop from the ledges ofthe rock, whose cold and rugged sides have condensed the vapour brought from the ocean ;these drops unite, and run down the acclivities in small veins, which in their course receiveothers, until by accumulation they" attain the character of a river. The quantity of rainthat falls must affect the state of a river. This varies in different districts ; in the neigh-bourhood of Paris it annually amounts to 18 or 20 inches; at Milan, in the north of Italy ,to 40, and in many mountainous districts to 90 or 100 inches; the summits of the Alps

Fig. 5ft3.

TIDE GAUGE.