134 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1918. 



records the revolutions of a small screw towed, by the ship and, 

 like a speedometer, registers the speed of the ship and the distance 

 run at any instant. 



The important features of modern practice in navigation may be 

 brought out by a brief account of methods now employed. In plan- 

 ning out in advance a long ocean voyage the navigator would first 

 lay down on a chart the track from port to port. For; this purpose 

 the monthly Pilot Chart, which is a Mercator projection, would be 

 preferred. It would be the navigator's object to adopt the shortest 

 route available, taking into account currents, winds, ice and other 

 undesirable features of high latitudes, as well as specified lanes of 

 traffic, and the intervention of land. Since the great circle route 

 is always shorter than any other, especially between ports far apart 

 in longitude and in high latitudes of the same name, a great circle 

 chart is often used, for on such charts a great circle appears as a 

 straight line. But since the track will be transferred finally from 

 the great circle chart to one on a Mercator's projection, which is 

 more convenient for purposes of navigation, the course may be 

 entered in the beginning on a Mercator's chart using the method 

 proposed by Airy for drawing approximate great circles. On a 

 Mercator's chart the track followed by a ship steering a continuous 

 course is a straight line (technically known as a rhumb line), and 

 since ships rarely, if ever, steer on great circles and, instead, follow 

 a series of rhumb lines like chords of the great circle track, differ- 

 ing successively one or two degrees in direction, it is desirable to 

 use a Mercator's chart upon which each such course appears as a 

 straight line. ^ : > 



Having thus planned the most advantageous general track to fol- 

 low, three methods are used to determine the position of the ship 

 at any time during the voyage. These are (1) projecting the track 

 on charts, (2) simple trigonometrical calculations based upon the 

 course steered and distance run as shown by compass and log, and 

 (3) astronomical observations with sextant and azimuth circle. 



Of these the first is generally least trustworthy owing to the un- 

 avoidably small scale of the charts. But when a ship approaches 

 or leaves a coast, chart sailing becomes obligatory and large scale 

 charts are available for the purpose. 



On leaving harbors, the so-called point of departure is found, 

 possibly by astronomical observations but preferably by sighting on 

 objects on shore as mapped on the chart of the port. In hazy weather 

 especially, a continuous line of soundings at fairly even distances 

 apart affords an additional control on one's position, and for this 

 purpose the sounding machines invented by Lord Kelvin, permitting 

 satisfactory soundings at speeds of 15 or 16 knots, are most useful. 



While in sight of land the course can be followed best by land 

 sights and soundings, a method of navigation usually referred to as 



