Observations on the Direction and Intensity of the Terrestrial Magnetic Force in Ireland, made by the Rev. HUMPHREY LLOYD, M.A., F.R.S., &c., Professor of Natural Philosophy in the University of Dublin; by Captain EDWARD SABINE, R.A., F.R.S., &c.; and by Captain JAMES CLARKE Ross, R.N., F.R.S., &c.

[With a Plate.]

THE observations which form the subject of the present communication were made during the years 1834 and 1835, in compliance with the recommendation of the British Association urged in the first and second Reports of its proceedings. Their main object has been to determine the direction of the lines of magnetic dip and intensity in Ireland, and to make a small, but it was hoped exact, addition to our knowledge of the laws of distribution of the earth's magnetism. The observations are threefold: first, observations of the horizontal part of the earth's magnetic force, as determined by the time of vibration of a needle suspended horizontally, after the method of Professor Hansteen; secondly, observations of dip, made in the usual manner; and thirdly, observations of dip and intensity at the same time, and with the same instrument, according to the method adopted by Professor Lloyd, and already submitted to the Association *.

I. Horizontal Intensity.

The instruments employed in the first series of observations were constructed after the model of that of Professor Hansteen. The needles are cylinders 24 inches long, and 13 of an inch in diameter, suspended by a few filaments of the silkworm's thread. They are inclosed in a small rectangular box, supported upon levelling screws, and having a tubular pillar screwed on at top for the silk suspension. At the bottom of the box is a divided circle, for the purpose of noting the arc of vibration: the temperature is observed by means of a small thermometer inclosed in such a manner as to avoid contact with the bottom and sides of the box. Before the commencement of the observations, the bottom of the box is to be rendered truly horizontal by means of the levelling screws on which it rests, and of a small spirit level with which it is furnished. The needle being then sus

*Fourth Report, p. 557. Transactions of the Royal Irish Academy, vol. xvii.

pended so as to hang near the bottom, its deviation, if any, from the horizontal position will be detected by its inclination to the surface. It is then to be slightly moved to one side or other in the brass stirrup by which it is supported, until it hangs truly parallel to the lower surface of the box; and when this adjustment is once accurately made, no further alteration will be required, unless the change of dip be considerable.

When an observation is to be made, the needle is raised or lowered by a small roller to which the silk suspension is attached, so that it may hang about midway between the upper and lower surfaces of the box. It is then drawn aside from the magnetic meridian through an arc of 25° or 30°, by a piece of brass wire inserted in the side of the box, and is allowed to oscillate. The registry of the oscillations is commenced when the amplitude of the vibration on either side of the meridian is reduced to 20°, and it is continued during 360 vibrations; the moment of the completion of every 10th vibration during that interval being noted by a chronometer. The amplitude of the final arc, or of the arc of the 360th vibration, is also observed; and the temperature of the air in the box, as indicated by the interior thermometer, is noted at the beginning and end of the observation.

It is obvious that in this manner seven intervals of time are obtained, each corresponding to 300 vibrations,-viz. the interval between the 0th and 300dth vibration, between the 10th and 310th, &c., and between the 60th and 360th ;-and the mean of these is taken as the result. But to this result several corrections must be applied.

1. The time as shown by the chronometer is to be corrected for rate; and accordingly the chronometer's rate must be determined from time to time by comparison with a good timekeeper, or by astronomical observations. In the present series the rate was observed at the commencement and end of each group of observations by the former and easier method. The amount of the correction due to rate is in most cases very small, the correction in the time of 100 vibrations corresponding to a daily rate of 2" being less than 0"01 with the slowest of the needles employed.

2. Professor Hansteen has applied a correction for the arc of vibration, so as to reduce the time to that corresponding to infinitely small arcs. The correction is investigated on the same principles as that usually applied to pendulum observations. It is however more complicated in its form; for, instead of a single series of vibrations, (as in the case of the pendulum,) we have here seven distinct series, each commencing from a different

arc.

The principle, however, seems hardly applicable in the present instance. It is assumed that the successive arcs of vibration decrease in geometric progression, as they must necessarily do if the resistance of the air be proportional to the velocity. This is found to hold good in the vibrations of the pendulum when the arcs are very small; but it is by no means true when they are so considerable as those in which the horizontal magnetic pendulum is made to vibrate. Where, however, the vibrations commence from the same arc, and the terminal arc does not much vary, the correction itself may perhaps be disregarded. In the following observations, in which the initial arc was 20°, the 360th or terminal arc was generally 210, and was in all cases included between the limits 1° and 4°. In such cases, then, the correction must be, nearly, a constant quantity; its application to the observed times is therefore nearly equivalent to their multiplication by a constant coefficient, and the ratio of the times (with which alone we are concerned in this class of observations) remains unaltered. For these reasons no attempt has been made to introduce a correction for the arcs in the following results; but the terminal arcs are given, so as to put the reader in possession of all the circumstances of the observation.

3. By far the most important correction is that due to temperature. If T' be the observed time of 100 vibrations corresponding to the actual temperature t', and T the corrected time corresponding to the standard temperature t, the correction is

T-TaT (t - t');

a bring a constant coefficient whose value is to be determined experimentally for each needle.

The following observations were made with the cylinders L(a), L (b), in order to determine the value of the coefficient a for each. The apparatus being inclosed in a large glass bell, the time of 100 vibrations of cylinder L (a), commencing with the arc of 10°, was observed at the mean temperature of the and when the air of the bell was heated artificially from below, by means of a spirit lamp. The final arc varied between 4° and 5°. The observations with cylinder L (b), were made in the bell without the apparatus. In this case no means were taken to observe with any accuracy the arc of vibration; and in order to reduce as much as possible any error arising from this source, the observations were continued in each instance until the arcs were reduced to the smallest appreciable, and the mean of the last five intervals of 100 vibrations then taken as the result. The chronometer's rate varied from + 06 to + 1′′·4

per diem, and had therefore no appreciable influence on the results.

The constant coefficient sought is to be calculated from the formula

in which tand are the two temperatures, and T and T the corresponding times of vibration. We find

Cyl. L (a). T' 242"-50, T-T' 1"45, t-t' = 23°•5

= =

a=000254

Cyl. L (b). T' 293"-44, TT' 1"78, t-t' = 24°•5

a=000248.

=

Stopping at the fifth decimal place, then, the coefficient for temperature for both cylinders is '00025. It is to be observed that these cylinders were made at the same time, and were therefore probably tempered to the same degree; and to this circumstance we may, with much probability, ascribe the close agreement in the values of the constant which determines the effects of temperature upon the force of the needle.

No observations were made to determine directly the effects of temperature upon the other needles employed in the course of these observations; and, in correcting the results obtained with them, the coefficient employed by M. Hansteen, viz.,

* A series of observations had been made with this cylinder, in the same manner and on the same days as those with cyl. L(a); but the results were unsatisfactory, some of them indicating an increase of force with increased temperature. Such contradictory results have been noticed by many observers, and are usually attributed to the disturbing effects of currents of air, determined by inequality of temperature.

1

00017, has been that adopted. The standard temperature (t), to which all the results contained in the following pages are reduced, is 60° Fahr.

4. All that we know of the diurnal variations of the intensity of the horizontal force, is due to M. Hansteen and Professor Christie. These writers agree in fixing the hours of minimum intensity at 10A.M. The intensity then increases, and attains its maximum, according to Professor Christie, at about 7 P.M. The amount of this maximum is 1.0024 in summer, the minimum intensity being unity; but this amount, as well as the hour of its occurrence, changes with the season. Of the law according to which the force varies between its two limiting values, we know nothing; and it is therefore impossible, in the present state of our knowledge, to apply a correction for these variations. It was proposed to evade this difficulty, in the ensuing observations, by observing at a fixed hour. To this limitation, however, it was found impracticable to adhere, and the results still remain uncertain by the amount of the diurnal change.

5. The variations of the magnetic force give rise to another and still graver class of errors. The least experience in observations of this nature is enough to prove that the horizontal intensity is, from some cause or other, subject to irregular fluctuations; and these fluctuations, like those of the barometer in our climates, are much more considerable than the regular horary changes. It seems probable that these variations in the intensity of the horizontal force are, like those in its direction, not local phenomena, but occur at the same time at places widely separated. To eliminate them from our results, therefore, it would suffice to have a regular series of observations made at some fixed station, cotemporaneous with those made at the different stations; and, if these be not very remote, we may assume that the variation of the observed force at each from its mean amount is the same as that observed at the same time at the fixed station. Unhappily these means of freeing the results from the admixture of what may be called accidental phenomena have not been attended to in the following, or indeed in any similar series of observations, and there is reason to believe that the errors due to this cause are the largest in amount of any by which the present series is affected.

The amount of these fluctuations, from day to day, may be judged of from the following specimen of a series of observations such as that alluded to, commenced by Captain Sabine in the month of June 1835. The apparatus in which the needle was vibrated was unmoved during the continuance of the series, and the needle remained permanently suspended. The height of the