Edinburgh New Philosophical Journal (1842 Edinburgh New Philosophical Journal edinburghnewphil32edin Year: 1842 - 1842 Hydromefrical Observations. 383 ance in tlie motion of the stream, und alters the velocity of the current, so tliat the result indicated by the elapsed time is more or less vitiated, and the mean velocity deduced from such data, is not, in almost any case, that which exists at the line of cross section. It is also impossible, by this method, to obtain a sufficient number of distinct and independent observations, applicable to each division of the stream, as the eddies and irr
Edinburgh New Philosophical Journal (1842 Edinburgh New Philosophical Journal edinburghnewphil32edin Year: 1842 - 1842 Hydromefrical Observations. 383 ance in tlie motion of the stream, und alters the velocity of the current, so tliat the result indicated by the elapsed time is more or less vitiated, and the mean velocity deduced from such data, is not, in almost any case, that which exists at the line of cross section. It is also impossible, by this method, to obtain a sufficient number of distinct and independent observations, applicable to each division of the stream, as the eddies and irregularities of the current which exist in all rivers, generally cause the lines passed over by the floats to cross and interfere with each other in such a manner as to destroy all connection between any given series of observations, and the several compartments of the river, whose mean velocity they were intended to ascertain. The superiority of the method which I am about to describe, consists in ascertaining the velocity of each portion of the stream, in the exact line in which the cross sectional area is taken. The instrument em- plo3-ed for this purpose is a modification of the tachometer of Woltmann, which is in general use in France and Germany, both as an anemometer and a hydrometer, being made of the degree of delicacy suited to the purpose to which it Is to be applied. In this instrument the velocity is measured by the current impinging on a vane and causing it to revolve, the number of revolutions made by the vane being registered on an in- dex, which is acted on by a set of toothed wheels. The construction of this beautiful instrument, and the manner in which it acts, will be best described by a reference to the accompanying cut, fig. 7., which is taken from
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