Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/169

Rh TERRESTRIAL MAGNETISM.] METEOROLOGY 159 be added, can only be reaped after a very large expendi ture of labour and money in organizing a comprehensive parochial scheme of observation, systematically and per sistently carried through and discussed. Further details regarding meteorological phenomena will be found in the articles ATMOSPHERE, BAROMETER, CLIMATE, HYGROMETRV OZONE, RAINGAUGE, SEA, and THERMOMETER. (A. B.) TERRESTRIAL MAGNETISM. 1. In the preceding portion of this article some account has been given of the influence which the sun and moon exert upon the air, the earth, and the ocean, their strictly tidal effects being left to be separately dealt with. The discussion of the influence of these bodies on what may be termed the movables of the earth will not be complete, however, without embracing an account of the changes which they produce in the earth s magnetism. An account of the earlier magnetic observations has already been given under the heading MAGNETISM, and our task will now be to give in the iirst place a description of the best and most recent instruments by which the magnetic state of the earth is determined, embracing therein observatory instruments, those adapted for travellers whether by land or by sea, and differential magnetometers. We shall next give a short account of the magnetic system of the earth and of its secular variation; and we shall then investigate the changes con nected with terrestrial magnetism depending on the sun and moon. In performing this task we shall be led to conclude that the sun s power is variable, and we shall therefore examine whether this con clusion is likewise borne out by strictly meteorological observations. Finally, we shall venture on remarks embodying a provisional working hypothesis, and our object will be gained if this should be found to suggest certain lines of thought to those interested in the subject which may lead them to examine and discuss the very great mass of observations at present existing. INSTRUMENTS FOR DETERMINING THE MAGNETIC STATE OF THE EARTH. (a) Observatory Instruments. 2. Declinometer. It is that end of the needle which points to the north magnetic pole of the earth of which the position is invari ably noted even when the observation is made in the southern hemisphere. The difference of this position from true geographical north denotes what is called the variation or declination (east or west) of the needle. East is often reckoned negative and west positive. The instrument by which this information is obtained is called the declinometer. The unifilar magnetometer, which is the form of declinometer now used, is described and figured in MAGNETISM, vol. xv. p. 238. 3. Dip Circle. The instrument by which the magnetic dip or inclination is observed contains a thin needle about 3 inches long, the centre of gravity of which coincides as accurately as possible with the axis of motion of the needle. The needle has two axles consisting of two very fine cylinders of hard steel standing at right angles to the plane of the needle, and great attention must be paid to keep these axles in a state of perfect polish and dryness. By means of these the needle can oscillate freely on two horizontal agate rounded edges, the one axle lying on the one edge and the other axle on the other. If the centre of gravity coincides exactly with the axis of motion, and if there be no adhesion or friction between the axles and the agate edges, the needle must settle into such a position that its magnetic axis lies in the true line of dip. The position of the ends of the needle is read by means of two microscopes which move round on a cross piece carrying verniers. To view the position of the lower end of the needle we move round the lower microscope until the cross wire in its field of view (extend ing in the line between the two microscopes) symmetrically cuts the extremity of the needle. The lower vernier is then read. The same process is repeated for the upper vernier, and the mean of the two readings is taken. This mean will accurately denote the position of the needle if the circle is properly set. The sources of error in a dip observation are (1) a want of symmetry in mass, the centre of gravity of the needle not being coincident with the axis of motion ; (2) the vertical circle being erroneously set ; (3) a want of symmetry in magnetism, the mag netic axis not being coincident with the axis of figure; (4) excen- tricity, the axis of rotation of the needle not passing through the centre of the circle ; (5) friction and adhesion of the axles as they rest on their agate supports. This last source of error is guarded against by taking great care of the axles, and by inserting them gently into a piece of cork before each observation ; the agate supports ought also to be rubbed with cork. Then, again, when the needle has assumed its position, before reading it is gently raised by means of a lifter, the handle for turning which is shown in the figure towards the right. It is then gently lowered, and this pro cess is repeated until no apparent change of position i produced by the operation. 4. We shall now describe a complete dip observation. The first point is to make the needle to swing in the plane of the magnetic meridian. In order to accomplish this, after levelling the instru ment, the verniers are set for 90, that is, for a vertical position of the needle. The whole instrument is now turned round its hori zontal circle until the extremities of the needle are bisected by the wires of the two microscopes, and the position of the vernier of the horizontal circle is then read. The needle is next reversed so that the microscope shall view its other flat side ; it is made vertical aa before, and tho position of the horizontal circle read once more. Next the face of the instrument is turned round 180, and the same two operations repeated. We have thus four readings of the hori zontal circle, and if we take the mean of these we shall have ascer tained with sufficient accuracy the position of that plane for which the needle is vertical. Now this plane must be removed 99 from the magnetic meridian, for in such a plane the horizontal magnetic force of the earth would have no resolved portion acting in the plane of the needle s motion, so that the needle would practically be under the sole influence of the vertical magnetic force, and would therefore point in a vertical direction. By this means therefore we obtain the magnetic meridian, and thus know the plane in which we ought to swing the needle. The needle must now be read in the following positions : (a) face of instrument east face of needle to face of instrument ; face of instrument west face of needle to face of instrument ; (7) face of instrument west back of needle to face of instrument ; (8) face of instrument east back of needle to face of instrument. Finally, the poles of the needle must be reversed, by rubbing them with powerful bar magnets in a direction opposite to that in which they were previously rubbed, and four observations taken corresponding to the above. The mean of the eight observations so obtained will give us the true dip. The turning round of the face of the instrument from east to west is made to counteract any error due to erroneous setting of the vertical circle. The reversal of the face of the needle is made to counteract any error due to the centre of gravity of the needle not being quite coincident in the direction of the needle s breadth with its axis of motion, and likewise any error due to want of symmetry of the magnetic axis. The correction for excentricity is made by reading both ends of the needle. Finally, the re versing of the poles of the needle is intended to counteract any error due to the centre of gravity of the needle not being coincident in the direction of the needle s length with its axis of motion. Dr Joule 1 has suggested a modification of the dip circle in which the needle is hung on fine threads on which it rolls instead of rest ing on agate supports. 5. Horizontal Force Magnetometer. The theory of the instrument for determining the horizontal component of the earth s magnetic force has already been given in the article MAGNETISM, vol. xv. pp. 238 sq., and the instrument is shown in two forms, ibid., figs. 28 and 29. The corrections necessary for accurate results are explained in a paper by G. M. &quot;W hippie (Proc. Roy. Soc., 1877). Instruments adapted for Travellers by Land. 6. Declinometer. For travellers by land the unifilar instrument ( 2), mounted on a tripod stand and duly levelled, is perhaps the- most accurate kind of declinometer. For this purpose it is furnished with a transit mirror by means, of which an image of the sun may be thrown into the field of view of the telescope, and the geographical position of the station as well as the apparent time of the observation being known an azimuth thus determined. In order that such an observation may succeed, the following points must receive attention. In the first place the axis of the mirror must be horizontal ; the adjustment for this is made by means of a riding level. Secondly, the normal to the plane of the mirror must be perpendicular to the axis. The adjustment for this is made by a screw attached to the back of the mirror. Take some object sufficiently elevated and reflect it into the telescope, getting the object bisected by the wire of the telescope. Then reverse the mirror in its bearings. If the object remains still bisected by the wire no correction requires to- be made, but if not the screw at the back of the mirror must be moved until the object is in precisely the same position in both observations. Thirdly, the line of collimation of the telescope must be perpendicular to the plane of the mirror. In order to obtain this there is a collimating eye-piece attached to the telescope by which the sun s light may be made to illuminate the cross wires. Now turn the transit mirror until the reflexion of the illuminated cross wires coincides with the wires themselves, in which case the line of collimation of the telescope must be perpendicular to the plane of the mirror. When this correction has been once made, note the circle reading of a small vernier which moves with the mirror and always set the mirror so as to give this reading. 1 Proc. Lit. and Phil. Society, Manchester, vol. viii. p. 171.