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cording to the width of the valley, which at some places is confined to narrow gorges, and at others widens out into pretty extensive haughs.

A great number of mills and other buildings are down entirely, there being not one stone or brick left on another; and a number more have portions carried away, leaving the floors and remaining walls in a very precarious situation.

Stones of a very large size, weighing, I believe, several tons, though I have mislaid the measurements which I took of them, have been moved a very considerable distance, and a number of steam-engine boilers have been carried down the valley, and left here and there like stranded vessels.

The most curious thing, however, that I remarked, was in the case of a large mill, which had stood in a gorge a very short way below the reservoir, in which there was not one brick of the walls left standing, and the steam-engine and fly-wheel were all upset and smashed, and yet, notwithstanding, the engine chimney, which is of brick and pretty tall, is standing erect and entire, although the pressure against it must have been very much increased, from the circumstance of the trunk of a large oak tree having been caught by it and been left lying across the valley, with its middle resting against the chimney, and its upper side pressed against by a large collection of stones and rubbish caught by it.

The loss of property has been computed at £250,000. This I should think a high estimate; but there is no gainsaying the lamentable fact, that not less than eighty-three lives were lost.

My researches, however, were confined almost exclusively to the mode of construction and the cause of the failure of the embankment, and scarcely extended to the scene of devastation below. Most probably the description will be felt to be obscure and unsatisfactory, unless to those acquainted with, and interested in, the technicalities of the construction of an embankment; but it may at all events suggest inquiries which, so far as it lies in my power, I shall be most happy to answer.

EDINBURGH, April 12, 1852.

Description of an Instrument by which the Variation of the Magnetic Needle can be determined with a greater degree of accuracy than has been attainable in Field Surveying. BY JOHN ADIE, Esq., F.R.S.E.* (With a Plate.)

SILVER MEDAL AND PLATE, VALUE TEN POUNDS, AWARDED 1852.

The very interesting and remarkable changes which are known to have taken place, and are constantly going on in the angular amount of the variation of the magnetic needle, on the surface of our globe, has in all ages claimed the attention of the observer of natural phenomena. A knowledge of the exact amount of this variation is not only useful and interesting in a scientific point of view, but is of the greatest consequence to the mariner, geographer, surveyor, and miner. The instruments that have hitherto been employed for the determination of this variation are of two classes,-First, those for the fixed observatory, as the variation transit instrument and others, from which very accurate results are obtained; secondly, those used by the traveller, mariner, and surveyor, as the azimuth compass, theodolite, and instruments of this class. To all of these this grand objection belongs, that you cannot collimate with the sights or telescope and the magnetic needle, which forms the basis of any correct determination of the amount of magnetic variation.

Before describing the instrument I have now the honour of laying before the Society, I may shortly state what is required in such instruments, and the practical methods employed to arrive at this. The most important is to know that the line of magnetism, or the magnetic axis of the needle, coincides with, or is parallel to, the line of sight to which it is to be referred.

In the azimuth compass all that is done is to place the centre of the steel needle as near as possible under the zero on the card, and there fix it. In the theodolite, the usual method is to place a straight edge along the line of the tele

*Read and exhibited 26th April 1852.

scope, and another on the compass ring, and bringing the edge of the one parallel to that of the other, a fine line is made on the compass ring, from which line the zero of the compass is deduced.

It will, I think, be obvious to all conversant with these matters, that with such rude means of placing the zero line of the card or needle parallel with the line of sight, no observation can be obtained in which any confidence is to be placed, even to the nearest degree of the circle, and the error may be much greater; in the azimuth compass, in particular, where the needle is commonly broad and heavy, the axis of magnetism may be very different from that of the needle.

To obviate these defects, the instrument I am about to describe (Plate XX.) was constructed. It consists of a magnetic needle, having its flat sides suspended vertically by an agate on a fine steel point. The case or box in which this needle is suspended for observation is in form exactly the same as the tube of the telescope of the theodolite, on which it is to be used, with openings in the upper side to admit the needle, and also for the admission of light to read its indications. Into these openings glass is fitted, to prevent the action of the air on the needle. It is provided with truly turned collars, the same as those of the theodolite telescope, which, when in use, rest in the Y's of that instrument.

At each end of the tube, and equidistant from the centre, at half the length of the needle, there is placed a glass diaphragm fixed in an adjustable ring, and having fine divisions cut on each side of the centre line. These diaphragms are each viewed by a powerful eye-piece placed at opposite ends of the compass box or tube. The centre division in each is then adjusted, that in turning the box round in the Y's this line remains fixed, or is truly in the centre of the collars on which it rests. The needle is now placed on its point, and the centre on which it is suspended is adjusted by screws provided for this purpose, till the ends of the needle also seen by the microscopes coincide with the centre lines on the diaphragms.

From what has been stated, it will be evident that we have now the means directly of collimating with the needle and

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the telescope of the instrument; for we know that the optical axis of the telescope is truly in the centre of turning on its collars. We also know, that the centre divisions in the compass tube are in the same line; and if we bring the points of the needle to coincide with these, the reading on the limb of the instrument will give the power of referring this magnetic meridian to any other point. But as it is nearly impossible to make a needle perfectly straight, and also to know that the axis of polarity coincides with the axis of the steel; and this, from irregularity in the structure of manufactured steel, giving it the power of holding a larger and more powerful charge of magnetic influence in one stratum than in another; I proposed to compensate for the want of perfect straightness at each observation or set of observations, by reversing the polarity of the needle, which affords the means of eliminating this source of error, and that from the form given to the needle; the second would be very small. I am, however, indebted to my friend Mr Swan for a suggestion, by which, by one simple operation, we get a perfect compensation for both sources of error, viz., in place of reversing the polarity of the needle, reverse the needle itself; in other words, observe first with the one surface of the needle up, and then with the other. By thus reversing the needle, you will have an equal and opposite error; the mean, therefore, will be the true line of the magnetic meridian, and it now only remains to turn the telescope of the theodolite into the true meridian line, and the difference between the readings of the magnetic meridian and the true will be the magnetic variation sought.

I have only been able to make three sets of observations with this instrument; the first with my friend Mr Jardine in the Observatory grounds, Calton Hill, from which we obtained 24° 42′ for the variation, and two series in the East Princes Street Gardens, which gave 25° 31', and 25° 27' These last agree very nearly, but differ from that obtained on Calton Hill. This difference may easily be accounted for from the magnetic influence of the hill, as we know that the rock of Arthur's Seat has the power of turning the needle quite round, causing the north end to point south, and the

south north; and the structure of the two hills do not greatly differ.

In conclusion, I give below some of the determinations of the magnetic variation, shewing the change that has gone and is still going on.

May 1804. Merchant Court,

W. 26° 30′ 0′′

Oct. 1808. Mr Roberts observed it in the park in front of
Watson's Hospital,

27 22 2

Oct. 1808. Mr Jardine found it, from observations on Calton

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From these observations it would appear, from that of 1804, the first we have record of, to 1812, the variation has been increasing to W.; and from those of the present day, that it has turned, and is now going E., or diminishing, which result is confirmed by the very delicate observations of Mr Broun made at Makerstoun Observatory.

Note. Since this paper was read to the Society, I have made an addition to the instrument, by which the line of the magnetic meridian may be laid down without the use of a theodolite. The instrument being placed in detached Y's, these may be turned till the needle point to the centre division; then remove one of the eye-pieces and substitute for it an achromatic object-glass; we will now have with this object-glass and one eye-piece left in its place, a telescope by which poles may be placed, the line joining which will be the line of the magnetic meridian.

Report of the Committee of the Royal Scottish Society of Arts, on Mr John Adie's description of an Instrument for determining the Variation of the Magnetic Needle.

The Committee have carefully read over this paper, and examined the instrument itself, which Mr Adie has exhibited along with the paper, and are of opinion as follows::

The variation of the magnetic needle has long formed an interesting and important subject of inquiry, and the object of the instrument now contrived by Mr Adie is to give the means of determining the amount of

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