Chap. VI.
FRANCE.
220
and the currents, resulting from the changes of temperature to which the Northern Ocean issubjected.
When the particles of a fluid are driven in one direction, as they are by the wind, theadjoining water is required to fill up the vacant space, and restore the equilibrium whichhas been destroyed; waves formed by this means, it was observed, and impeded by theproposed line of timber cones, might render the water on the opposite side comparativelytranquil, but the force at the two extremities would be so great, that it would be impossiblefor a vessel to enter or approach theharbour. The violent impulse givento waves in a storm continues longafter the gale which has produced ithas subsided, and an oscillatory motionremains, and for some time works uponthe great mass of water, which in thisinstance, it was said, would be suffi-ciently powerful to displace any arti-ficial contrivances in carpentry. Theagitation produced by the wind doesnot, however, affect the waters at anygreat depth ; their surface is alone dis-turbed, for in the roughest weather at80 feet the sea is always tranquil.
The force of the tides was still morelikely to be injurious; for, being in-terrupted in their current, it was urged,the water would so accumulate inheight, when obstructed by the pier,that it would have sufficient force toremove it from its position; and thecurrents which set in so violently atthe bottom of the ocean would bepowerful enough to displace any ar-tificial obstruction. The two polarcurrents, which float mountains ofice from the frigid to the temperateregions, and which are encoun-tered as far as the fortieth degree oflatitude, were also supposed to becapable of driving away a barrier, orbreakwater, placed in their course. Itwas also maintained, that no obstacleson land could turn aside the currents,or oppose a barrier to the grand move-ment of the ocean, and it was vain toattempt it: but these arguments didnot weigh with Cessart. When histimbers were all firmly bolted together,and secured by the best means he couldcontrive, he commenced his prepar-ations for floating them.
The execution of these immensetimber constructions was confided toship carpenters, and the whole wasplanked and bolted together in thesame manner as the sides of a vessel:to float them seems, perhaps, the mostingenious part of the contrivance,though the method adopted was by nomeans new, for casks have been em-ployed on the English coast for cen-turies, to buoy masses of stone re-quired to form a mole or terminationto a jetty. Pliny has described themeans adopted by the ancients to float heavy bodies, and in all probability the systemthey practised has never been entirely forgotten: in this instance the casks seem to havebeen preferred to vessels; they were more under the control of the engineer, and could bemore easily detached.
Q S
—) □ a
Fig. 239.
FRAMING OP CONE.