Scale 1/10000 is equivalent to 7.30 inches to one nautical mile. Scale 1/20000 is equivalent to 3.65 inches to one nautical mile. Scale 1/40000 is equivalent to 1.82 inches to one nautical mile. Scale 1/50000 is equivalent to 1.46 inches to one nautical mile. Scale 1/80000 is equivalent to 0.91 inch to one nautical mile. Scale 1/100000 is equivalent to 0.73 inch to one nautical mile. Scale 1/200000 is equivalent to 0.36 inch to one nautical mile. Scale 1/400000 is equivalent to 0.18 inch to one nautical mile. Scale 1/1000000 is equivalent to 0.07 inch to one nautical mile. Scale 1/1200000 is equivalent to 0.06 inch to one nautical mile. For use in measuring distances on large scale charts the length of one or more nautical miles is usually drawn on the chart, and sometimes scales are also given in other units. On British charts the nautical mile scale is divided into tenths (that is, cables of 100 fathoms or 600 feet length); on the American charts into quarters and eighths. Where the scale covers more than one mile the fractional divisions are shown only for the left-hand mile and the zero of the scale is placed between this and the full mile scale, so that with dividers the full miles and fraction may readily be taken off. The nautical mile in the United States is taken to be the length of a minute of arc of a great circle on a sphere whose surface equals that of the earth; this definition makes the nautical mile equal 6080.27 feet. Lecky adopts 6080 feet as the nautical mile. The length of the actual minute of latitude on the earth's surface increases from 6046 feet at the equator to 6108 feet at the poles, an increase of about one per cent. It is, however, this somewhat variable unit of length which is ordinarily used in scaling distances on the sailing charts. On small scale charts there is usually a border scale entirely around the chart, conveniently subdivided; this serves the double purpose of facilitating the plotting or reading of positions by latitude and longitude and of furnishing a scale of minutes of latitude for use in measuring distances. On a mercator chart this scale of course varies with the latitude and it must be referred to in the mean latitude of the distance to be measured. In general practice the minute of latitude is taken as equal to the nautical mile.