IX.

Observations on the Progress of Bodies floating in a Stream: with an Account of some Experiments made in the River Thames, with a View to discover a Method for ascertain→ ing the Direction of Currents. By JAMES BUrney, Esq.

HAVING

stream faster

rent.

AVING frequently noticed, that the heavy craft on Heavy bodies the River Thames, during a calm and without the assist- float down a ance of oars or of towing, made a progress faster than the than the cur stream of the tide on the surface, it led me to make inquiry as well into the fact as concerning the cause, and gave rise to some experiments, which, with the ideas they suggested, are here set down; no otherwise according to method than being in the order they occurred.

On questioning the men belonging to several barges, Laden barges which, unaided by wind, oar, or towing, were floating with float faster than light ones. and overtaking the stream, they all agreed in the general fact, as a circumstance familiar to them. They said likewise, that a laden barge made greater progress than a light barge; and this was corroborated by the evidence of the boats attached to them being drawn after them; for the barges overtook the moving water so fast as to have good steerage way. They attributed the difference in favour of a laden barge, to her having (as they expressed it) more hold of the tide than a light barge: by which it appears, that they supposed the stream of the tide was stronger underneath than on the surface. Adhesion to the atmosphere may retard the surface, except when the current of the atmosphere (the wind) goes in the same direction with the current of the tide; and then it may occasion an acceleration.

Monday, July the 18th, I went on board a barge half This is not ow laden, which was floating down the river, but with steerage ing to a more rapid under way, between Putney and Chelsea bridges. I conjectured current. the rate of the tide to be a mile and a half per hour: there was a very light air of wind in a direction contrary to the VOL. XXIV-SEPT. 1809.

E

stream

The surface of a stream an inclined plane.

Wherry out

barge.

stream of the tide: but the barge, without

any assistance of oars or towing, passed on, overtaking the stream, and her boat was towing astern. I fastened a riband to the end of a stick, and immersed it in the water about 20 inches, which was as low as the lowest part of the barge's bottom, and therefore sufficient to have shown, by the direction of the streamer, if the barge had been impelled forward by superior velocity of the under current, as in that case the streamer would have gone before the stick; but the streamer tended towards the stern, and was drawn after the stick whence it was evident, that the barge's progress exceeded that of the stream underneath as well as on the surface, and that this excess was acceleration produced by some other cause*.

As by the general law of gravitation the heaviest bodies descend with most velocity in a yielding medium, so it appears to be with bodies floating in a stream. The surface of a stream or current of water is not horizontal, but an inclined plane, and the inclination of the surface produces the current. Thus, when, by the attraction of the Sun or Moon, the sea is raised in some parts, it becomes depressed in others, and the water, seeking to regain its level, flows in a current from the superior parts.

The barges on the river in a calm therefore slide downward with the stream, and also on it.

A friend of mine in a wherry going to pass under Lonstripped by a don bridge, being closely preceded by a coal barge, was apprehensive of receiving damage from collision with the barge when under the bridge; but the waterman said the barge would shoot far enough ahead when she came to the indraught of the arch. And it happened accordingly; for

Under currents produced by local circumstances.

*No part of what is here said contradicts any received hypothesis concerning under currents. Some under currents proceed from visible causes, as when the wind blows for a length of time in one direction towards a coast, especially if it is an embayed coast, whereby the waters are accumulated and the surface near the shore is raised above the general level, till the pressure of the increased weight forces back the water underneath. Under currents, where the causes are not visible, may be supposed to be caused by inequalities in the bottom, in the same manner as eddies are caused by the projecting points of a coast interrupting the general course of a stream.

the

the barge, arriving first within the sterling heads, shot away from the wherry about 200 yards, by the superior momentum she acquired in the increased déclivity.

A pressure perpendicular to the horizon applied to a body Perpendicular and oblique

floating on a horizontal surface acts as increase of weight, pressure acting

having the effect only of making the body to which it is ap- on a floating plied swim deeper, or occupy more space in the water. An body. oblique pressure, not: strong enough to submerge the body, affects it in two directions; one downwards in the manner of weight, to which the body yields to a certain and definite extent; the other horizontal, in which direction the body continually gives way to the pressure. Almost every person has experienced the readiness of a boat to glide from under him, on putting his first foot in her. These two effects of an inclined pressure are separately in proportion to the whole pressure, one as the sine, the other as the cosine, of the angle of incidence is to radius.

stream.

If to a body floating on a horizontal surface a pressure is Applied to a applied in a direction making with the horizon an angle of barge on the 89° 59′, the proportion of the pressure which would act horizontally is to the whole pressure, as the sine of l' is to radius. And this proportion is T of the whole pressure. In like manner, if the surface inclines 1' from the true horizontal level, weight applied to a body floating on that surface will give an impulse towards the declining part of the surface equal to T of the weight applied. Consequently, a barge having in her 100 tons weight, floating with the stream where the declivity of the surface is 1', will receive an impulse towards the declining part of the surface equal to nearly 65lbs. which is little short of what is esti mated to be the average pull of a horse.

current.

Hence it seems naturally to follow, that two pieces of Indication of wood, equal in size but differing in weight, being placed in the course and the water near to each other, would show if there was a curstrength of a rent, by the heavier wood separating from the lighter in the direction of the stream. Likewise, that the quantity of separation in a given time might afford a measure for the strength of the current. And it is probable, that this would be found true in a smooth and equal running stream, where no interruption was caused by the wind.