GENERAL NAYIGATION, 11 



yards on a large-scale chart, whereas on one of small scale the same 

 amount of displacement means a large fraction of a mile. 



Distortion of printed charts. — The paper on which charts are 

 printed has to be damped. On drying distortion takes place from the 

 inequalities of the paper, which greatly varies with different paper 

 and the amount of the original damping, but it does not affect navi- 

 gation. It must not, however, be expected that accurate series of 

 angles taken to different points will always exactly agree when care- 

 fully plotted on the chart, especially if the lines to objects be long. 

 The larger the chart the greater the amount of this distortion. 



Mercator's chart. — Observed bearings are not identical with 

 those measured on the Mercator chart (excepting only the bearings 

 North and South, and East and AVest on the equator) because the line 

 of sight, except as affected by refraction, is a straight line, and lies 

 in the plane of the great circle, while the straight line on the chart 

 (except the meridian line) represents, not the arc of a great circle, 

 but the loxodromic curve, which on the globe is a spiral terminating 

 at the pole, or, if the direction be East and West, a circle of latitude. 



The difference is not appreciable with near objects, and in ordinary 

 navigation may be neglected. But in high latitudes, when the 

 objects are very distant, and especially when lying near east or Avest, 

 the bearings must be corrected for the convergence of the meridians 

 in order to l)e accurately placed on the IVIercator cliai't. A\hich repre- 

 sents the meridians as parallel. 



On the poly conic chart, since a straight line represents ( within the 

 limits of 15 or 20 degrees of longitude) the arc of a great circle or 

 the shortest distance between two points, bearings on the chart are 

 identical with observed bearings. 



The Mercator projection is evidently unsuited to surveying, for 

 which purpose the polyconic projection is used by the Hydrographic 

 Office and the Coast and Geodetic Survey, and the gnomonic projec- 

 tion by the British service. 



Notes on charts should always be read with care, as tliey may 

 give important information that can not be graphically represented. 



Buoys. — It is manifestly impossible to rely on buoA^s always 

 maintaining their exact positions. Buoys should therefore be re- 

 garded as warnings, and not as infallible navigating marks, especially 

 when in exposed places: and a ship's position should always, when 

 possible, be checked by bearings or angles of fixed objects on shore. 



Gas buoys. — The lights shown by gas buoys can not be implicitly 

 relied on; the light may be altogether extinguished, or, if intermit- 

 tent, the apparatus may get out of order. 



Lig-hts. — All the distances given in the light lists and on the 

 charts for the visibility of lights are calculated for a height of 15 



