cuity and practice, to consider this application of the principle under three separate cases.

CASE I. Where the level of the mines is known to be the same, but the direction is doubtful.

In this case it is only necessary to apply the magnets in one of the mines in a given position, and then at the same horizontal level of the other, (say on the floor of the mine, or at any particular distance above), to try the deviations of the compass at moderate intervals, from side to side, directly across the headway, when, from the observation of the greatest deviation, the point nearest to the magnets may be ascertained. One precaution, however, will be necessary. If the direction of the compass-needle be not exactly at right angles to the position of the magnet, then the greatest deviation will not occur at the nearest point but a little beyond, towards the place where the action of the magnet is most perfectly tangential *. Hence, it will be generally advisable, not only to adjust the needle of the compass, by a directing magnet, as near as may be to a right-angular position with the magnet (as shown by the known magnetic position of each mine), but also to repeat each experiment with both poles of the magnet alternately presented to the compass, so as to obtain the difference of deviations of the two poles at each position of the compass. In order to conduct this experiment with the greater facility and uniformity of effect, the simple apparatus represented in Fig. 7, Plate V. of preceding volume, formed out of a piece of board, may be useful.

C is the compass, and D a directing magnet sliding in a groove, which is so adjusted as to its distance from the compass, that the needle may assume a position parallel to the straight edge

• As this circumstance might in some cases occasion a considerable error, if only the deviation by one pole were taken, it would be useful to have the means of ascertaining the extent of error capable of being thus produced. This would be given by a table of the powers of the magnets, calculated from the sines of deviation instead of the tangents, which would shew the distance of the magnet from the compass, as placed at right angles to the needle in its deviated position. At great distances, however, no material mistake can arise from the neglect of this source of error,-the powers of the sines and tangents being so nearly the same in deviations under 5 or 6 degrees as to be within the ordinary limits of error of observation.

of the board a a, when that edge is placed parallel to the interposed substance, the thickness of which is to be ascertained. The adjusting magnet is then fixed in its place by the thumbscrew s, so as to secure a constant influence, whilst the parallelism of a a with the wall of the mine is preserved; and, in order to facilitate and secure the preservation of that parallelism, a deal plank may be placed directly across the mine against the extremity of the working, and supported at the required level, which would not only serve as a table for the compass apparatus, but having nailed upon it a straight batten near and parallel to the edge, it would enable the experimenter to bear the compass from side to side of an irregular working, and to preserve in every part of the distance the parallelism of the straight edge a a. Fig. 8. Plate I. illustrates this arrangement.

Suppose T to be the working of a mine, and T' the approximating working, and it is required to ascertain the thickness of the intermediate mass of rock R. The case of magnets M being placed against the middle of the wall, in one of the headways, the plank P is placed at the same horizontal level on the opposite side, when the compass apparatus, guided by the batten e e, being carried from side to side, will shew the greatest deviation in the position a. Let the magnets be now turned with their opposite pole towards the compass, (the time for doing which is to be indicated by signal, as about to be suggested), and the deviations tried a second time across the mine. If the greatest deviation be in the same position as before, that may be assumed as the nearest point; if not, the mean of the two positions must be taken.

It is not necessary to describe the experiments more minutely, nor to enter into the management of the apparatus for all particular conditions; for to do so would swell this paper to an inconvenient length, whilst the details would be easily anticipated by a very little practice. It may be proper, however, to mention, that where the deviations on the removal of the compass continue to increase to the very extremity of the board, or to the side of the headway, it will be necessary to cause the magnet to be removed in the opposite direction, so that the maximum deviation may be satisfactorily obtained.

CASE II.-Where the direction may be certain, but the level doubtful.

Here the magnet and compass being placed on the known line of direction, the compass is moved progressively upward, from the floor to the roof of the mine, to determine the greatest deviation, which, it is evident, will be on the same horizontal level as that of the magnet.

A result, perhaps more satisfactory, might be obtained by the use of a needle, suspended like a dipping needle, and traversing in a vertical plane. Such a needle being brought into a vertical position by a small directing magnet, would shew by its deviations the position of corresponding level.

CASE III.-Where both level and direction may be uncertain.

In this instance, the magnet being fixed, and the plank placed on the floor of the mine, the compass apparatus is to be carried from side to side, till the place of the greatest deviation is ascertained, or the middle between the two positions of greatest deviation produced by the different poles. This will give the directional position. The vertical needle being now tried throughout a vertical line drawn through the directional point, will shew by its greatest deviation the horizontal level of the magnet, which, being at the same time in the directional line, will indicate the nearest attainable point between the magnet and compass. The vertical needle, in this and similar cases, may be guided in its transit by a board, with a straight edge, or pillar of wood, placed vertically, so as to preserve its parallelism of position at every altitude.

It may be proper here to call to mind, that the chief object of the processes now described, is the determination of the thickness of solid substances not otherwise measurable, and not primarily at least, the determination of direction and levels; for the application of the observation of deviations to these latter purposes is only expected to afford an approximation, in some cases, indeed, a very important approximation, to the truth. Where the distance to be measured is small, say only 8 or 10 feet, or in any case in which the deviations amount to 3 or 4 degrees,—the line of direction may be very accurately determined by the middle point between the maximum deviations

of the opposite poles of the magnets, and especially if each maximum be itself determined by the mean of equal deviations on both sides of the maximum; in this case I have frequently determined, at moderate distances, the very point on the cross plank, from whence a line drawn at right angles will pass through the magnets, consequently the nearest possible distance between the magnets and compass. Where the deviations, however, are very small, as it will then require a considerable change in the position of the compass, laterally, to produce any sensible change in the deviation,-the line of direction, or the level, will be proportionally less certain, though the nearest distance may be sufficiently ascertained.

As the person who conducts any experiments on the foregoing principles will have to direct the application of the magnets, with any changes that may be requisite as to their poles or position, whilst the experiment is in progress, an arrangement of directions by signal will be necessary; and this may be very well accomplished by the blows of a hammer or mall.

The distance at which any sound can be heard through solid substances, will depend both on the nature of the concussion, and the quality of the vibrating mass. Through earth, gravel, or other loose materials, the transmission of sound is very li mited and uncertain; but through solid uniform rock, directions may be communicated by signals to very considerable distances. In solid and uniform sandstone, the pick, I understand, may be very well heard through a distance of fifteen or sixteen yards, whilst the vibration produced by a blast may be perceived at very great distances beyond. The working of coal by the pick may be generally heard sixty or eighty feet, I am told; though no rational determination of the thickness can be made from the nature or intensity of the sound,—for a change in the structure of the bed, or the interposition of an increased quantity of moisture, may produce as great an alteration in the sound as might be expected if the distance were doubled! The blow of a mall upon an iron wedge set in the rock, may be heard much farther than the stroke of a pick, or the blow of the heaviest hammer on the face of the rock,-and this will form a convenient arrangement for the purpose of signals.

VOL. XIII. NO. XXV.-JULY 1832.

I

By the use of two elements,-number of blows and an extra interval of time in different parts of the series, an abundant code of signals might be arranged within the limits of not more than eight or nine blows in succession *.

* The following may serve as an illustration of the method proposed for signalizing by means of blows. The number might be increased, and the signals modified, by employing scratchings with the point of a pick-axe, which can be heard at considerable distances, as well as by the method of blows.

3. Apply the magnet (N. pole)
where you are; as near as
you can tell opposite me, at
the floor of the mine.

4. Reverse the pole of the magnet,
and apply it to the same
spot.

5. Neutralize the magnets. 1, 2. Apply the N. pole of the magnet to the middle of the working, at the floor of the mine.

1, 3. Carry the magnet to the right on the same level. N. B. The distance in yards to be indicated by the number of successive blows.

1, 4. Ditto to the extreme right of the mine; same level.

2, 1. Ditto to the left,......so many yards.

2, 2. Ditto to the extreme left of the mine.

2, 3. Raise the magnet vertically from the floor, so many feet. 2,4. Raise it to the roof, directly upward.

3, 1. Depress it to the floor, directly
downward.

3, 2. The experiment is finished.
3, 3. The experiment is not finished.
3, 4. Do you understand?