for plotting on a smaller scale, as 40 or 44 feet to the inch, single lines would alone be necessary except in very wide streets. The scale of th is generally the

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largest adopted in surveys of any description, all larger scales being used merely for details of engineering works; but as an exceptional case may be mentioned a small survey made for plotting to the scale of 3 feet to the inch, in which the offset measurements were taken correctly to th of an inch, with steel tapes, instead of to full inches, as it was specially required to illustrate the accuracy of a few lengths of tunnel where a through meeting had been formed.

Section 3. TELEMETER SURVEYS.

Telemeter surveys differ from chain surveys in that the distances, within a range convenient to the telemeter used, are obtained by observation with instruments instead of by chaining (for Telemeters and Telemetrical Observation, see Chapter I., section ii., pages 10 to 24). For instance, if the power of the telescope and arrangement of the distance wires are such as to give correct distances in sights taken between 100 feet and 1000 feet, any distance exceeding 1000 feet has to be taken. in portions, while any detached distance less than 100 feet must be measured with a tape or offset staff; correspondingly also when the powers of the instrument are limited to a range between 66 feet and 660 feet.

The general method of procedure, as regards triangulation and offsets, is substantially the same as that adopted in chain survey, unless the telemeter is accompanied by instrumental means for observing angles. The need of the occasional minor operations previously

H

explained on pages 87 to 94 is greatly reduced, as small brooks, sheets of water, etc., are not obstacles to telemetrical observation; and again, if Porro's method be adopted, the necessity for checking by tie-lines is also reduced, as each telemetrical observation with three wires affords a check on its own accuracy.

While telemetry possesses great advantages over chaining in reducing monotonous labour with at least an equal amount of precision, it must be remembered that it is more specially suited to surveys in which the distances are many and rather long, and the offsets comparatively few.

In this, as in all such substitutions of mechanical for manual work, the men employed must be more intelligent and capable. The staff-holders must not only be capable of holding the staff truly vertical, but of choosing the points necessary, ranging themselves correctly in any direction, taking the offsets well and truly at right angles to any direction or survey line, and also of roughly recording them, as their memories cannot be depended on for several points at a time; to do this well and without loss of time, they must also be capable of understanding and applying the elementary methods of procedure in survey work, and be worthy of some confidence. Work of this class. is hence best entrusted to assistants and pupils in preference to the stray working men and boys generally employed in chaining.

The form of record for telemeter surveys is generally similar to that used in chain surveys, the principal difference being that the record itself is more lengthy and extended, from three telemetrical readings being entered in place of every single measured distance, whenever