494 On the Water in an Aqueduct, obstructed by collections of Air.

to pass water, or any heavy fluid, through it in the direction. from A to B, the end A being elevated the distance a b above B, c d being a horizontal line. It is evident that the water being let into the end at a will pass and fill the pipe to e, displacing all the air with which the pipe, being open to the atmosphere, was previously full. Flowing over the curvature e in a stream or column less than the bore of the pipe, it fills the curvature at f without displacing the air previously contained in the descending section from e to f. This air is thus shut up, and cannot pass from the pipe in any direction without passing under the water, which, from its inferior specific gravity, is impossible. The water, continuing to flow over the flexure e, rises from f to g, and flowing over this flexure the same thing is repeated, as to the air from g to h, which took place at the flexures e and f. Rising from h until it attains some point, i for example, at which the sum of the perpendicular heights of the ascending columns c e, fg, &c. is equal to the height of the column a b. That is, if we suppose the air to be un-elastic and void of weight, but as this is not true in fact, the air will be condensed in a greater or less degree according to its volume and the height of the columns of water opposed to it. In consequence of this condensation, the water will rise, as shown in the figure, to k and m for example, and the weight of these columns being added to the effective force of the column a b, produces a rise of the water to some point, n, in the flexure h n. There is then a perfect equilibrium in the opposing forces, and the water can flow no farther. This equilibrium may be expressed generally by a b + c d be

in which a is the perpendicular height of the water in all the descending flexures; b its density; c the perpendicular height of all the inclosed air; d its mean density; and e the perpendicular height of all the ascending columns of water.

Several writers on Hydrodynamics have noticed the obstruction which air often presents to the passage of water in bent tubes; but in the works that I have had an opportunity of consulting, the authors appear to regard the air as collecting in the high parts of the tube, and partially closing its bore, thus diminishing without totally obstructing the discharge. This is quite different from the effect of the arrangement which I have attempted to explain. Those, however, who are acquainted with this subject will recollect the Zurich machine for raising water, invented many years since, as owing its efficacy to an arrangement which the air and water take

in a spiral tube, very similar to that stated in the preceding part of this paper.

As the aqueduct at the mill-dam was more or less bent through its whole course, the flexures being considerable at the creeks under which it passed, it appeared to me certain that it was partly filled with air, and that this alone interrupted the flow of water. On opening small holes into it in several places, air rushed out in great quantity; still, however, the water did not flow at the reservoir, and as it was impossible to get at the bendings in every part of the pipe without the labour of uncovering it wholly, the design of freeing it from air by piercing it with small holes was suspended. A forcing pump was then coupled to the upper end of the pipe, and water, which had been heated in the worm tube of a distil house, in the vicinity, was forced into it. The pump was furnished with a valve loaded with a weight equal to a column of water 80 feet high, and a very small opening made from the aqueduct into the reservoir at the mills, so that the water passing slowly through the whole length of the aqueduct was there discharged. The object of this apparatus was to produce an absorption of the air by bringing it in contact, under heavy pressure, with water which had parted with some of its air by being heated; as these conditions are known to be favourable to the absorption of air by water. The pumping was continued about ten days, and the quanti ty of water used may be taken at 20 hogsheads; when the pump was taken off, and the aqueduct opened into the fountain. The water was then found to flow at the reservoir, discharging as much as was due to the head. This discharge has continued uninterruptedly to the present time, about five months. There can be no doubt but much air was absorbed, its presence in the aqueduct being indicated, when the pumping was commenced, by its throwing a stream of water out of the pipe, on which the loaded valve was placed, whenever the weight was removed from the valve. The quantity of water thus thrown back was much too great to have been produced from the elasticity of the water or the lead pipe, and it diminished daily, having almost ceased before the pump was taken off.

General Intelligence.

Influence of Copper on the Magnetic Needle.-At the sitting of the Royal Academy of Sciences, at the Institute, on Monday, the 7th of March, M. Arago made an important communication to the Academy on the subject of a curious and valuable discovery recently made by him relative to the influence of copper over the movements of the magnetic needle. M. Arago had already communicated to the Academy, in November last, the discovery that the number of oscillations made by the needle in returning to its proper direction after it had been removed from it, may be considerably diminished by means of a copper ring, that metal producing exactly the same effect on the oscillations of the needle, as would result from its immersion into a liquid. This fact is so very evident and remarkable, that it is surprising how it so long escaped the observation of scientific men. M. Arago is now enabled to show, in a new light, the influence which copper exercises over the magnetic needle. This he has succeeded in doing, by causing that action which was before a retarding action, to become, in its turn, an impelling action. This requires explanation. As there is no action in Nature without reaction, if a copper cylinder in a fixed position is capable of acting on a needle when in motion, we may be certain that, by a reciprocal influence, the cylinder, if it be rendered moveable, will act upon the needle when stationary. But the question was, to ascertain the laws of that action, and how it was liable to be modified in proportion to the bulk or distance of the objects, which M. Arago has succeeded in demonstrating by means of a piece of clock-work made entirely of copper, and which, when it is put in motion, causes to turn with considerable rapidity a magnetic needle, which is placed above it, and separated from it by glass plates, in such a manner, as to remove all cause of error arising from the influence of the external air. Copper is not the only substance that has the property in a greater or less degree. This fact had been already ascertained in some degree by Coulomb. That celebrated academician had been led to the discovery by a succession of experiments, which, however, did not appear to him sufficiently conclusive to decide him to make them public; the more so, as they were strongly contested by several distinguished men of science. What Coulomb left undetermined, M. Arago has succeeded in proving in the most evident and

satisfactory manner; for in addition to the cscillations just mentioned, he has effected an angular deviation of two minutes in the needle. It is impossible to raise any doubt on this point, for he has calculated the deviation by means which would have enabled him to distinguish any deviation, even not exceeding that of a few seconds. These discoveries of M. Arago are not only curious in themselves, but they are capable of being rendered of great utility. In the first place, they afford a means of avoiding the errors which were necessarily committed by those, who, in order to ascertain the magnetic intensity of the earth, had occasion to make use of compasses, in which the needle was surrounded by a copper ring: secondly, they afford a means of remedying the disadvantages resulting from the mode of suspension of the magnetic needle, adopted in those instruments which are desired to be rendered very sensible. Since the improvements introduced by Coulomb, the needle has been suspended by an untwisted silken thread, and then left to itself. It is clear that a needle suspended in this manner possesses a perfect power of motion, but that very power of motion presents the serious disadvantage of prolonging the oscillations in an embarrassing manner, so much so, as to render it sometimes necessary to wait for twenty-four hours before the needle becomes perfectly steady. This valuable discovery of M. Arago furnishes a certain means of putting a sufficiently speedy stop to the oscillations of the needle, by means of a copperplate properly fixed to the instrument, and that without changing, in the slightest degree, the definite direction of the needle. It will also be possible, by the same means, to diminish the oscillations caused in the needle by the motion of the sea, and consequently to make magnetic observations even when the sea is strongly agitated; an advantage which has been for a long time past a great object of desire, but which has hitherto been sought for in vain.-Lond. Lit. Gaz.

New Method of Bleaching Flax and Hemp.-The flax and hemp being broken and cleaned, and laid in bundles of less than a pound weight, are to be immersed for six hours in a solution of slaked lime, of the consistency of white wash. When the lime is discharged from the fibres by clean water, the flax is then to be boiled in water, with about 4 oz. of pot or pearl ashes, for nearly six hours, fresh water being supplied when necessary.

During the process of boiling, the flax must be taken out and put back into the alkaline solution, to disturb its colour

ing matter, and when it is sufficiently boiled, it will feel slippery between the fingers. It is now to be washed in clean water, and again put into a solution of lime as before, repeated agitation being employed. It may now remain in the solution at rest for six hours, and when it is washed with clean water, the fibre will be left pure, but with a slight yellow tinge.

In order to remove this tinge, plunge the flax in a weak solution of sulphuric acid and water, and after keeping it there for three hours, it will be found to be of a pure white, and when passed through the hackle is ready for use.-Lond. Jour. Arts.

A Singular Scintillation of the Stars.-On Sunday the 12th Oct. 1823, at 4, A. M. Baron Zach (Corr. Astron. vol. ix. p. 301) observed at Genoa a very remarkable scintillation of the stars, which astonished all who saw it. The stars seemed to throw out sparks and jets of flame with surprising rapidity and vivacity. The same effects were seen by every person, and also through an achromatic opera glass, so that the phenomenon must have had its origin in the atmosphere, and was, we think, owing to a want of homogeneity in the aerial medium, similar to what takes place in mixing alcohol and water.

Baron Zach observed another very curious fact, which surprised him more than the preceding phenomenon. When the observer fixed his eye steadily upon any star, its scintil lations became more settled and tranquil; but the stars seen at the corner of the eye, or by indirect vision, became more disorderly or rapid in their scintillations. Baron Zach states, that he has sought in vain all our works on optics for an explanation of this effect.

The laws of indirect vision, upon which this curious fact depends, have been investigated and explained by Dr Brewster, in a paper on the Eye, read before the Royal Society, Edinburgh, 3d Dec. 1822. In that paper he has shown, that bodies seen by indirect vision alternately vanish and re-appear, even when seen with both eyes; and that they assume colours different from their natural ones, certain colours vanishing in preference to others.-Brewster's Jour.

Musical Thrush of Brazil.-We first observed in these woods the notes of a greyish-brown bird, probably a thrush, which frequents the bushes and ground in damp low woods, and sings with numerous repetitions through the musical scale, from Hito A (of the German scale), so regularly, that not a