Whirlpools are produced among the Orkney Islands by the actions of winds and currents ; but boats, it i.e said, peas over the spots in safety, a log of wood or q bundle of straw previously thrown into the water being sufficient to arrest its revolving motion.
The circular or spiral motion of the water, which constitutes a whirlpool or eddy in a river, is produced by flexures of the banks or contractions of the bed ; in consequence of which the current, instead of continuing parallel to the general direction of the river, is turned obliquely towards the middle : the particles of water between this oblique current and the banleity which the waters from the upper part of the river are reflected, are acted upon by forces in different directions; and the centrifugal force resulting from the curvilinear motion causes the centres of the whirlpools to be on a lower level than the general surface of the water in the river.
Let A n be the distance between the two banks of a river at a part where a contraction of the bed begins to take place, and let a 45 be the narrowest part of the channel, in the vicinity: the water in part arrested by the bank A a, rises above the general level of the water in the river at that place, and being reflected, will be made to take an oblique direction, as a c ; at the same time, the velocity in the con tracted section becomes, by the laws of hydrodynamics, greater than that of the river above A E. Then the particles of water within the space a c e, rushing towards a c, the surface within that space becomes depressed, and the particles about D descend into the apace by their gravity. It follows that there is a constant tendency of the waters from to towards c a, and from c towards a es besides the currant in the direction a c; and by the action of the forces in these directions the revolving motion takes place. Whirlpools are continually being formed
in this manner, and are carried to some distance down the river by the general current.
Whirlpools may in like manner be formed at the same time, below b, on the opposite bank of the river, if this should have a similar form to the bank between A and c ; or the stream a c may be reflected from E, should there be a contraction at that place, and whirlpools may be formed in the enlargement beyond, as shown in the diagram. Precisely in like manner are formed the whirlpools or eddies at the shoulders of the piers of a bridge, when the breadth of the river is so much con tracted as to cause its surface above the bridge to be considerably higher than the surface below. Under these whirlpools the bed of the river must evidently sustain less pressure than takes place on the parts about them : consequently, the water under the bed, acting hydrostatically upwards, may lift up the earth and stones,?and thus undermine the piers ; or it may blow up the piles driven for the formation of dams. By this cause the accidents which occur in hydraulic operations are frequently produced.
Inequalities in the depth of the bed of a river must evidently give rise to vertical whirlpools by the reflexion of the water from the ascending slopes; the particles then take an oblique direction upwards, so as to rise like a wave above the general surface also, a sudden depression of the bed will produce a vertical whirlpool in the lower part, nearly as the horizontal whirlpools before mentioned are supposed to hare been formed.