PHYSICS, EIGHTEENTH CENT. MAGNETISM. 337 



so that he had to place a counterpoise on the south end. He was 

 advised to make an instrument for the express purpose of finding at 

 what angle the needle would dip below the horizon when its movement 

 in that direction was unrestrained. He thus ascertained the dip to be 

 at that time 70 50'. The dipping-needle, as it is called, is now con- 

 structed with great improvements, .so as to obtain the greatest possible 

 accuracy in its indication. Fig. 163 shows the dipping-needle in one 

 of its simpler forms. The needle turns in a vertical plane on a hori- 

 zontal axis which passes through its centre of gravity, so that only the 

 magnetic forces affect its position. A graduated circle is so fixed that 

 the amount of the dip may be read off by the position of the points 



FIG. 164. THE SPIRIT-LEVEL. 



of the needle. The instrument is adjusted by means of a spirit-level, 

 which is shown on a larger scale in Fig. 164. When the instrument 

 is used it is first placed so that the needle stands vertically, and in 

 this case the plane of the graduated vertical circle will be perpendicular 

 to the magnetic meridian. The degrees which the index on the hori- 

 zontal graduated circle marks are observed, and the upper part of the 

 apparatus is then turned through an angle of 90, when the plane of 

 the dipping-needle will coincide with that of the magnetic meridian, 

 and the dip may then be observed. 



Newton, Hooke, Huyghens, Halley, and others devoted some atten- 

 tion to magnetism. Halley in 1698 and 1699 made two voyages ex- 

 pressly to investigate the variation of the compass. He traversed 

 various parts of the Pacific and Atlantic Oceans, and made so large a 

 number of observations, that from these and some results which others 

 had obtained he was able to form a chart of the variation of the com- 

 pass. Many such charts have subsequently been made ; and in order 

 to bring before the reader a general view of the variation, Fig. 166 is 

 given as the sketch of such a chart. It represents, as will be seen, 

 first a chart of the earth on Mercator's Projection, in which the meri- 

 dians and parallels of latitude are shown by parallel straight lines. 

 These are the fainter white lines in the diagram ; the more distinct 

 white curved lines are the magnetic curves. It will be noticed that 

 these form two sets ; one set which is in a direction in general parallel 

 to the equator, and another set which cross the former at right angles. 

 These last lines indicate the variation, for a compass at any place would 



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