Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/171

Rh TERRESTRIAL MAGNETISM.] METEOROLOGY 161 FIG. 22. Azimuth Compass. manner that the wire above mentioned shall bisect the sun s visible disk. There is a totally reflecting glass prism which throws into the eye-piece an image of the scale of the graduated card, so that the observer, having first bisected the 1 sun s disk by the wire, must next read the division of the scale which is in the middle of the field of view. He thus obtains a reading of the sun s position ; let us call this 100. From this, knowing the geographical posi tion of his station and the time of the observation, he may de duce an azimuth ; let us imagine that this is 70 W. Thus a reading of 100 corresponds to a position 70 W. Suppose next that the instrument is so ad justed that when the magnetic axis of the magnet is between the eye-piece and the wire the reading is 0. It is thus clear that the magnetic meridian is 100 removed from the position 70 W. Let us imagine that the instrument is so graduated that this denotes a position 30 E. We have thus obtained the mag netic declination. If the vessel be at rest the plan generally adopted is to take the reading of the sun when rising and also when setting ; a mean between the two will give that which cor responds to a geographical meridian. 14. Fox s Dip Circle. This instrument, contrived by Robert Were Fox, is more especially useful for observations at sea. In this case it must be placed on a gimball stand and duly levelled before commencing the observation. The following are the peculi arities of this instrument : (1) the needles have two fine pivots or axles which are inserted into jewelled sockets ; (2) in order to avoid parallax there are two graduated circles, the one farther from the eye than the other, and when reading the needle the eye is to be so placed that pre cisely the same reading shall be given by both circles, the true position of the needle being thus obtained ; (3) there is a rubber made of bone or ivory and roughened, the object of which is to rub a prolongation of the socket on the back of the instrument, the fric tion which this rubbing causes enabling the needle to find its true position ; (4) to avoid as much as possible all effects due to friction and adhesion, the entire socket arrangement may be turned round. The axles of the needle are thus compelled to be in contact with a different set of particles. An other way of varying the suspension is to use a magnetic deflecting arrange ment attached to the back of the apparatus. Suppose that a reading of the position of the needle so de flected is now taken. Next reverse the position of the deflecting arrangement, which is done by turn ing a movable circle attached to this arrangement 180 round; let the position of the needle be again read. On the hypothesis that the needle is equally deflected on opposite sides of its true position in these two observations, the mean reading will give the true dip. The principle of the method of observing with this circle is precisely the same as that already described for observations on shore with an ordinary inclinometer. 15. Fox s Intensify Arrangement is merely a modification of that introduced by Lloyd, and already described in 7. 1 (8) Differential Magnetometers and Self -Recording MagnctoyrapJis. 16. In addition to determinations at fixed intervals of time, it is a point of much interest and importance to keep a continuous record of all the magnetic changes which take place at a few selected stations. This is accomplished by means of differential magneto meters. It is, however, necessary to continue to use absolute instruments side by side with differential magnetometers, because the latter (with the exception of the declination instrument) are badly fitted for recording changes of long period, such as the secular changes of the horizontal and the vertical force. The reason of this will presently be seen. 17. Early in the history of such instruments it was found that hourly observations were exceedingly laborious, and attempts were made to construct a set of self-recording magnetometers. The first set of such instruments which were brought into systematic opera tion were those devised and constructed by the late Charles Brooke, which have been at continuous work in the Greenwich Observatory since 1848. In 1857 John Welsh devised a fresh set of self-record ing instruments, and introduced them into the Kew Observatory. These, with certain slight modifications, have formed the type of instruments supplied to a large number of magnetic observatories all over the globe. 18. As we cannot conveniently record changes of dip by a differential instrument, changes of vertical force are measured instead by a balance or vertical force magnetometer. We have thus in a differential system, whether adapted to eye observation or to continuous photographic registration, three instruments, namely, the declination, the horizontal force, and the vertical force mag netometers or niagnetographs as the case may be. The most recently constructed instruments are adapted both for photographic registration and for eye observation through a telescope. The advantage of eye observations is that we see what is taking place at the very moment of its occurrence, whereas we only obtain the photographic record some time after the changes to which it relates have actually happened. We shall therefore describe (a) the three instruments of the Kew pattern as adapted to eye observations ; these instruments as adapted to continuous registration by photography ; (7) the method of determining their scale coefficients ; (5) the method of determin ing the temperature coefficients of the force instruments. 19. Kew Instruments Eye Observations. Fig. 23 shows us these instruments arranged in the relative positions recommended by Lloyd so as magnetically to interfere with one another as little as FIG. 23. Kew Instruments. possible. We are supposed to be viewing the whole from the south. No. 1 to the right is the declination instrument, No. 2 that for the horizontal force, and No. 3 in the distance behind the central pillar (No. 4) the vertical force magnetometer. Figs. 24, 25, 26 give us the details of these three instruments in the same order as above. Connected with each instrument there is a circular mirror, or rather two semicircular mirrors, made of perfectly plane glass. One semicircular half of each mirror is attached to the magnet and moves with it, while the other half is firmly attached to the marble slab. Each magnet is enclosed in a gun-metal case with windows of perfectly plane glass ; each gun-metal case is covered with a glass shade ; and the whole is air-tight, and capable of exhaustion. Each magnet too is provided with a copper damper with the view of checking its oscillations. In fig. 23 v.-ill be seen two pillars of smaller size. The right-hand pillar carries a tele scope, with a scale attached, to record the position of the declination magnet. The scale is reflected from the semicircular mirror moving with the magnet, and the position of this reflected scale as viewed in the telescope indicates the position of the magnet. The optical arrangement for the other instruments is similar, except that the vertical force mirror has a horizontal and not a vertical axis. The telescopes for viewing the force instruments arc attached to the left-hand pillar of smaller size. 20. The Declinometer - (fig. 24) consists of a magnet about 5 inches long suspended by a silk thread freed from torsion as completely as 2 For a detailed acccunt cf the Kew magnetographs, see British Astociation Reports. 1859. XVI. 21