MAP 01' STREET DISTANCES.
London, 28th May 1851. Sin—All the suggestions made to you by correspondents as to marking distances on a map of London for the purpose of ascertaining cab fares, seem to me to have great objections. Both on the plan of the circuiter and on the plan of dividing London into square miles, there will be distances on the map both practically and mathematically incapable of being measured ; for instance, on the latter plan, the diagonals of the squares. The following plan seems to ins to obviate this objection. Rule the map from left to right at distances equivalent to one mile apart. Rule the map from top to bottom at distances equivalent to three quarters of a mile apart, and draw one set of diagonals to the rectangular parallelograms so formed. The length of these diagonals will be equivalent to one mile and a quarter; and if the centres of these diagonals be marked by a dot, measure- ments can be made from the corners and centre of every parallelogram in eight different directions, with perfect accuracy. A very few words printed on the side of the map would explain the mode of measuring to the meanest
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