6y 



pole. The parallel of 48° of latitude is but two-thirds the diameter 

 of a great circle, and the parallel of 60° is only one-half the size ; and 

 it is on these parallels tliat the mariner steers his ship, when his port 

 is east or west of his ship, if he adopts any other system of navigation 

 than that of great circle sailing. If he sails north or south, he sails on 

 the arc of a great circle, whatever system he employs, because the 

 meridians, or north and south lines, are great circles dividing the earth's 

 surface into equal parts. Also the mariner who sails east or west on 

 the equator, sails on the arc of a great circle, the equator being a great 

 circle ; but, in all other cases the mariner who sails by Mercator's chart 

 takes his ship by a circuitous route. If he does not sail in the 

 direction of one of the cardinal points, he sails on what is technically 

 called a rhumb line ; which on the chart is represented as a straight 

 line, but on the globe is a spiral, uialung endless revolutions round the 

 poles. Thus Mercator's sailing conducts the ship by a circuitous route, 

 as compared with great circle sailing. It was adopted on account of 

 its simplicity, and not on the supposition that it conducted the vessel 

 by as short a route as great circle sailing did. By Mercator's sailing a 

 ship may be navigated by one course throughout the voyage. For 

 instance, if the ship and her port be both in the latitude of 50° north, 

 and the port west of the ship, she could then reach her port by sailing 

 a due west course throughout the voyage. But not so by great circle 

 sailing ; she would then have, if in north latitude, first to steer north of 

 the west, and constantly vary her course to the left, till at length she 

 would reach her port by a south-westerly course. This fact requires 

 explanation ; the ordinary mariner cannot be made to comprehend how 

 it is possible that, by varying his course continually, he will reach his 

 port by a shorter track than by sailing one course all the voyage. It 

 is generally supposed, if the place A be west of B, that B must be east 

 of A. This would be the case if this earth were a plane, but it cannot 

 be so on the surface of the globe. East or west are local or relative 

 terms ; theji have no existence in space, but refer only to the locality 

 in which we are situated. If I say that A is west of me, I imply that 

 the position of A is at right angles to the meridian, or north and south 

 line of the place in which I am situate. Now if all the meridians were 

 parallel to each other, then a line at right angles to one would be at right 

 angles to all. And the meridians of all places are represented by parallel 

 lines on the chart ; and so it is that, by referring to the chai't, it appears 

 that if A be to the west of B, then B must be the east of A. But on the 

 globe, the meridians, or north and south lines, meet at both poles in 

 angles varviug with the latitude and the difference of longitude ; conse- 



