KANGE OF TIDE. 



through a cork. The point which is nearest to the eye, and which 

 corresponds to the crest of the wave in the former example, con- 

 tinually occupies a different point of the worm, and continually 

 advances towards its extremity. 



This property has been prettily applied and illustrated in 

 clocks for the chimney-piece or console. A round rod of glass, 

 twisted so that a ridge in the form of a screw is produced upon 

 its surface, is inserted in the mouth of some figure, such as a 

 lion or a dolphin, which being supposed to discharge water, 

 forms a fountain. The extremity of the glass rod concealed 

 within the mouth of the figure, is fixed on the axis of a wheel, 

 to which a continual motion of rotation is imparted by the 

 works of the clock, and the other end is concealed in the vessel 

 designed to represent the basin, or reservoir, of the fountain. 

 The constant rotation of the twisted glass rod produces the 

 appearance of a progressive motion from the mouth of the figure 

 to the reservoir, as already explained in the example of the 

 cork-screw, and the rod of glass appears like a stream of water 

 continually issuing from the fountain, and falling into the 

 reservoir. 



To return to the phenomena of the tides, it is necessary to 

 observe that there is, nevertheless, a real progressive motion of 

 the water directed up the course of tidal rivers, and upon the 

 fiat strands of bays and inlets. This, however, is not the pro- 

 gressive motion of the tide-wave, but that of the water falling 

 from the height to which it has been raised, as it might flow 

 down the side of a declivity. 



17. The difference of level between high and low water is 

 affected by various causes, but chiefly by the configuration of the 

 land, and is very different at different places. In deep inbends of 

 the shore, open in the direction of the tide-wave and gradually 

 contracting like a funnel, the convergence of water causes a very 

 great increase of the range. Hence the very high tides in the 

 Bristol Channel, the bay of St. Malo, and the bay of Fundy, 

 where the tide is said to rise sometimes to the height of one 

 hundred feet. Promontories, under certain circumstances, exert 

 an opposite influence, and diminish the magnitude of the tide. 

 The observed ranges are also very anomalous. At certain places 

 on the south-east coast of Ireland, the range is not more than 

 three feet, while at a little distance on each side it becomes twelve 

 or thirteen feet ; and it is remarkable that these low tides occur 

 directly opposite the Bristol Channel, where (at Chepstow) the 

 difference between high and low water amounts to sixty feet. In 

 the middle of the Pacific it amounts to only two or three feet. 

 At the London Docks, the average range is about 22 feet ; at 



143 



