MARINER'S COMPASS. 



netism, are brought almost all to the same 

 point. After the discovery of that most 

 useful property of the mag-net, or load- 

 stone, viz. its giving a polarity to harden- 

 ed iron or steel, the compass was many 

 years in use before it was known in any- 

 wise to deviate from the poles of the 

 world. About the middle of the sixteenth 

 century, so confident were some persons 

 that the needle invariably pointed due 

 north, that they treated with contempt 

 the notion of the variation, which about 

 that time began to be suspected. How- 

 ever, careful observations soon discover* 

 ed, that in England and its neighbour- 

 hood, the needle pointed to the eastward 

 of the true north line ; and the quantity 

 of this deviation being known, mariners 

 became as well satisfied as if the compass 

 had none ; because the true course could 

 be obtained by making allowance for the 

 true variation. 



From succeeding observations it was 

 afterwards found, that the deviation of 

 the needle from the north was not a 

 constant quantity, but that it gradually 

 diminished, and at last, namely about the 

 year 1657, it was found that the needle 



pointed due north at London, and has ev&r 

 since been going to the westward. 



The azimuth compass differs from the 

 common sea compass in this, that the 

 circumference of the card or box is di- 

 vided into degrees ; and there is fitted to 

 the box an index with two sights, which 

 are upright pieces of brass, placed dia- 

 metrically opposite to each other, having 

 a slit down the middle of them, through 

 which the sun or star is to be viewed 

 at the time of observation. See AZI- 

 MUTH. 



The figure of the compass card, with 

 the names of the 32 points or winds, are 

 given, Plate Miscel. fig. 10. As there 

 are 32 whole points quite around the 

 circle, which contains 360 degrees, there- 

 fore each point of the compass contains 

 the thirty-second part of 360, that is, Hi 

 degrees, or 11 15'; consequently the 

 half point is 5 3?' 30", and the quarter 

 point 2 48' 45". 



The points of the compass are other- 

 wise called rhumbs ; and the numbers of 

 degrees, minutes, and seconds, made by 

 every quarter point with the meridian, 

 are exhibited in the followin 



