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flect the needle at all, but will weaken the directive force which the terrestrial magnetism exercises upon it. The same effect may be produced by acting in this way upon both poles at once. By different modifications of the same principle, the needle may be made to assume any required direction with respect to the meridian.

In all these cases the terrestrial directive force being diminished, any extraneous force acting laterally or tending to make the needle deviate will act with a greatly increased power, and consequently the phenomenon of the daily variation may be thus exhibited in its magnified effects.

Mr. Barlow's principal course of experiments consisted in observing the position which a delicate horizontal needle took at different hours in the day, when under the influence of reducing magnets. By means of these the needle was first retained in its natural direction, and the deviation observed through the course of three days. It was then successively, by the same means, made to assume a variety of different directions: such as the north end to the south; the poles East and West; and in short all the various intermediate positions. In each the deviations at different hours were observed during two or three days.

The general result we will give in the author's own words.

"While the north end of the needle is directed to any point from the south to the N.N.W. its motion, during the forenoon, is towards the left hand: advancing therefore to some point between the N.N.W. and the N: and while it is directed toward any point between the N. and S.S.E. it passes to the right hand, that is still to some point between the N. and N.N.W: the south end of the needle at the same time passing of course to some point between the S. and S.S.E: so that it would seem that there ought to be some direction between those limits, namely, between N. and N.N.W. and S. and S.S.E.; in which the daily motion is Zero, or at least, a minimum. But whether this is a fixed direction during the year, or whether it has any vibratory motion as the sun changes its declination, or even during his daily course, is a question which cannot be decided without a much longer course of experiments than those I have here the honour to present."

Mr. B. proposes a question as to whether the direction of this line of no variation is the same in different parts of the world. He also observes, that the amount of the deviation does not depend entirely upon the moment when the heat of the sun is greatest, as has been generally imagined.

Another inference which he thinks may be fairly drawn from these observations, is that the daily change is not pro

duced by a general deflection of the directive power of the earth, but by an increase and decrease of attraction in some point situated between N. and N.N.W., or between S. and S.S.E.

Mr. Barlow also tried the effect on the dipping needle by the same means, but his results have not hitherto been such as to exhibit any determinate law.

At an early stage of the foregoing experiments, Mr. B. was surprized by observing a remarkable anomaly between the daily changes of the needle when observed in his garden, and when in the house. In certain positions of the needle towards the East and West, the daily motion, although it proceeded with the same determinate uniformity in other cases, yet it took place in different directions: passing in the one instance from the East or West towards the South, and in the other towards the North, at the same corresponding hours of the day, the motion in both instances being equally distinct, regular, and progressive.

He then considers the various causes which may have tended to the production of this effect. And upon the whole considers it most probable though not absolutely proved, that the solar light is the principal agent concerned.

The anomaly was also observed in a completely different and independent set of experiments made by Mr. Christie. To Mr. C.'s paper we now proceed.

No. 24. On the diurnal deviations of the horizontal needle when under the influence of magnets. By Samuel Hunter Christie, Esq. M.A. Fellow of the Cambridge Philosophical Society, and of the Royal Military Academy.

In these experiments, Mr. Christie adopted a somewhat different arrangement from that employed by Mr. Barlow. It was an inference from his theory that the effects would become more unequivocal if the magnets employed to diminish the directive force were placed, not in the same horizontal plane with the needle, but in the line of the dip passing through the same centre. In this way then they were arranged in the following experiments.

The needle was first arranged in its natural position. It was observed every half hour from six A.M. to eleven P.M. The point at which it seemed most stationary, or about N. 0° 20′ E. was assumed as zero, from which the deviations were reckoned. These then were traced regularly through the time mentioned.

A similar set of observations was next made, with the directive force still further diminished..

The results of all the observations are given in the form

of tables. They have also been delineated by the author in a diagram, representing the deviations as ordinates to a curve, the distances between them being the intervals of time, and the axis corresponding to the position of zero. The general course of the deviation is slightly to the E. at between 7 and 9 A.M. then westerly, the maximum usually from 1 to 2 P. M., and easterly again towards night.

Many small anomalies and irregularities were observed, which Mr. C. was inclined to attribute to changes in the atmosphere, and particularly to its electric state...

Another set of experiments was next made, in which the position of the needle was inverted.

For one day the needle was observed very closely at intervals of half an hour, or sometimes less, from 2 A. M. to 11 P.M.

A graphic delineation of these results is given also. In a general view, the author observes,

"The principal irregularities to be observed here take place in general from about three o'clock in the afternoon to seven o'clock; and we might almost be led to infer, from this circumstance, that there are two causes in opposition to each other, producing the deviation; and that these being nearly equal at this time, alternately preponderate; but the observations are too limited to allow of our drawing such general inferences from them. We shall see, from observations in other positions of the needle, that irregularities very frequently take place during the afternoon.

"In this position of the needle, the morning deviation is at first towards the E., and afterwards towards the W., the same as when the north end of the needle pointed N., the needle tending towards the same point in the two cases, but in contrary directions. This is what we might expect from a simple change in the direction of the terrestrial force, but the observations which I shall presently describe, clearly point out both a change of direction and of intensity."

Mr. Christie then proceeds to an investigation of the nature of the force acting upon the needle when under the influence of the magnets; which from its nature is not susceptible of abridgment, and from its length we cannot, consistently with our necessary limits, give entire. The object of it, however, is to shew what would be the resulting effect upon a needle thus circumstanced, supposing a change were made in the intensity of the force acting upon it, with respect to the points of stable and unstable equilibrium: and again, what effect would be produced by a change in the direction of the force thus acting. These deductions were put to the test of experiment by altering, in an appropriate manner, the

positions of the reducing magnets, and observing the corresponding effects on the needle.

These effects were compared in an extensive and elaborate series of observations, with the actual changes which take place in the points of equilibrium in the needle, under the influence of the terrestrial and reducing forces, throughout the day. The author's conclusion, from this comparison, is as follows:

"The agreement in the general character of these, with that of the observations from five o'clock till eight, will clearly lead us to infer, that the changes observed in the positions of the points of equilibrium during that time, arose from a diminution of the terres trial forces, as well as a change in their directions: had the agreement in all the observations themselves been complete, we must have inferred that diminution of intensity was the sole cause of the changes."

And again, after the sequel of the experiments, he concludes thus:

"Comparing, then, the whole of the preceding observations with these effects, it is evident that the changes which take place cannot be explained by a change in the directions alone of the terrestrial forces, but that their characters agree as nearly as we can possibly expect, with the effects that would take place from an increase of intensity at the time that the direction deviated towards the west we are therefore led to infer, that such an increase of intensity must take place in the terrestrial force during the time of the westerly deviation. The change of intensity during the day has been already ascertained by the observations of Hansteen on the vibrations of a needle very delicately suspended, but, in the present state of our information respecting the magnetical phenomena of the earth, the series of observations which I now present will not, I trust, be considered without interest, although they should at first sight appear only to confirm results obtained from previous observations."

Mr. Christie now proceeds to mention the effects of a change of temperature in the reducing magnets. Under a hot sun these effects were conspicuous; and the increase of temperature was found to diminish the power of the magnets. A portion of their power seemed to be permanently destroyed. Hence it might be supposed, that these changes of temperature were the principal, if not the only cause of the deviations. This idea Mr. C. put to the test, by observing the changes in the points of equilibrium, in a situation where the magnets were exposed to very small variations in temperature; the changes were repeatedly found to be in opposition

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to those which would have been produced by a change of temperature in the magnets alone.

Some anomalies had been observed, both by himself and Mr. Barlow, between the effects within doors and without. This he conceives to have arisen from the difference in the changes of temperature in the two situations.

"Of this," says Mr. C., in conclusion, "I only feel that degree of doubt which should always be entertained until a fact is established. If such observations as I have given were continued for a length of time, particularly those near the east and west, I certainly expect that they would lead to important conclusions respecting the causes of the diurnal variation, and I regret, that as I have not the time to devote to them myself, I must leave them to be made by others possessed of more leisure. Should they be undertaken, the necessity of ascertaining, in the first instance, the effects which changes of temperature have on the forces of the magnets employed, and of observing the temperature of the magnets themselves, when the directions of the needle are taken, is here clearly pointed out. The striking effects which I have seen to arise from a change of temperature in the magnets, have certainly led me to adopt the opinion, that temperature, if not the only cause of the daily variation, is the principal. This was the opinion of Canton, but he could not by it account for the morning easterly variation. I might here offer some conjectures on this subject, but, as it is not my intention at present to enter fully into the general question of the cause of the daily variation, I will defer them, at least until I shall have ascertained the precise effect of changes in the temperature of magnets."

Under the head of magnetism, we must also class No. 26, on the apparent magnetism of metallic titanium, by W. H. Wollaston, M. D. V.P.R.S.

In this paper the investigation described in our account of the former part of the Transactions for the year, is continued. By applying more delicate methods, Dr. W. found that titanium, after being freed from all apparent mixture with iron, still exhibited a very slight magnetic power. This was so very weak, compared with that of cobalt and nickel, that it seemed more probably to be due to a minute admixture of iron, which it must be difficult to detect in an analysis, however carefully performed, than to so improbable a circumstance, as that titanium alone should possess such a very slight degree of magnetism, whilst all other known magnetic metals possess it in a very sensible degree.

On the interesting subject of electro-magnetism, we have, in the present part of the Transactions, two papers, No. 13, On a new phenomenon of electro-magnetism, by Sir Humphry Davy, Bart., P.R.S.; and

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