Page:Small-boat sailing; an explanation of the management of small yachts, half-decked and open sailing-boats of various rigs; sailing on sea and on river; cruising, etc (IA smallboatsailing01knig).pdf/217

 will be seen that the lines representing the meridians on the extreme right and left are graduated, that is, marked off in degrees and minutes of latitude. These graduated meridians can be used as a scale for the measurement of distance upon the chart. But as degrees of latitude, though everywhere of equal length, on Mercator's chart are drawn longer as the poles are approached, each particular section of the graduated meridian can only serve as a scale for the regions on that latitude.

Thus if we wish to measure the distance between two places on the chart, one of which is in 50° North latitude and the other in 60° North, we must use as scale of miles that part of the graduated meridian lying between those limits. The following is the quickest method of procedure. Open the dividers so that the two legs are on the two places on the chart. Then with the dividers measure off this distance on the graduated meridians, so placing the dividers that the mean latitude between the two places (which would be 55° North in the above example) lies half-way between the two legs of the dividers. The distance can now be read off the graduated meridian, in degrees and minutes, a degree containing 60 minutes or nautical miles, which are to English miles roughly as seven are to six. On coasting charts, where small areas only are represented, an ordinary scale of nautical miles is sometimes drawn, which serves to measure distances with sufficient accuracy.