COM PASS 



391 



The directive property of the magnet seems to 

 have lieen unknown in Kniope till the 12th century. 

 It appears, however, on very good authority that 

 it was known in China and throughout the East 

 .generally at a very remote period. The Chinese 

 annals assign its Ouooverj to the year 26.S4 H.<'. , 

 when they say an instrument for indicating the 

 ^outh was constructed by the Kmperor Ilu-angti. 

 The earliest reference to the making of magnet> is 

 in a Chinese dictionary of 121 A. I)., where lodestone 

 is defined as 'a stone with which an attraction can 

 be given to the needle ; ' but this proiierty of the 

 lodestone could not fail to have lieen observed at a 

 very much earlier time. At first the 

 ( 'hiuese would appear to have used the 

 compass exclusively for guidance in 

 travelling by land, and we near of their 

 using it by sea only somewhere about 

 -HOO A.I). According to one account, a 

 knowledge of the compass was brought 

 to Europe by Marco Polo on his return 

 from his travels in Cathay ; but as against 

 this, the late Mr William ChappelT, in a 

 letter to Nature (June 15, 18/6), pro- 

 duced evidence to show that we owe the 

 .appearance of the compass in Europe in 

 the 12th century to independent discovery, 

 and not to importation from China. 



The ordinary mariner's compass is made 

 up as follows : The needle is a magnet- 

 ised strip of steel, or in the better com- 

 passes a number of thin strips magnetised 

 separately and then bound together. 

 This is balanced so as to swing horizon- 

 tally on a fine pivot. Fastened to the 

 upper surface of the needle and swinging 

 with it is a circular card marked with 

 the thirty-two 'points,' and having the 

 point marked N immediately over the 

 end of the needle that is attracted to the 

 north. The pivot on which the needle 

 swings stands up from the bottom of a 

 copper bowl, which has a glass covering 

 to protect the contents from wind and 

 weather. The compass-bowl is made of 

 copper (a good conductor of electricity) in order to 

 damp the vibrations of the needle. For the needle 

 in moving induces currents of electricity in the 



pass Imwl is sometimes filled with Hpirit, but the 

 additional friction of the liquid interferes with the 

 sensitiveness of the needle. The bowl is supported 

 in (! imhals (q. V.)HO as to remain horizontal in all 

 positions of the ship. The whole arrangement v* 

 placed in the binnacle, situated when possible in 

 the fore and aft line, and having provision for the 

 placing of lamps to illuminate the compass-card by 

 night 



The compass-card is represented in fig. 1. The 

 four cardinal directions or ' point - ' are marked 

 N. E. S. \\ .. and there is a convenient notation for 

 intermediate points. Thus the point midway be- 



** 



** 



Fig. L 



copper bowl, the electro-magnetic forces of which 

 tend (according to Len/'s Law) to oppose the motion 

 producing them. For the same purpose also the corn- 



Fig. 2. 



tween N. and E. is called NE., that again between 

 N. and NE. is called NNE. (read north-north-east), 

 and so on. The remaining sixteen points, such as 

 NiE. (read north by east), are named on a plan 

 which will be readily made out by an examination 

 of the figure. These thirty-two points are often 

 further subdivided into halves, quarters, and even 

 eighths, and then we have NE., and so on. When 

 the ship lies in the magnetic meridian, and points 

 to the north, the N. point on the card is directed 

 towards a black line, called the lubl>er's line, 

 marked on the inside of the compass-bowl. Boxing 

 the compass means reading off from memory the 

 thirty-two points in order going round either 

 way. 



Modified forms of the compass are in use on land 

 by surveyors and miners, and need not be further 

 alluded to here. 



By way of summarising the qualifications of 

 a good compass, we shall describe the l*est compass 

 in use Sir William Thomson's, paten ted in 1876: 

 A thin aluminium ring (fig. 2 ) is connected by silk 

 threads with an aluminium IM>SS, which rests on an 

 aluminium cup having a centre piece of sapphire 

 poised on an iridium point (fig. 3). Instead of a 

 single needle there are eight thin strips of steel 

 fastened to the silk threads as shown in the figure. 

 The thin paper rim bearing the points is divided at 

 intervals so that the contractions and expansions 

 due to change of temperature may not produce 

 warping of the aluminium. The whole arrange- 

 ment weighs only 200 grains or so, and thus there 

 is very little tendency to flatten the point of the 



