Aug. 29.] THE NEW ZEALAND GAZETTE. 2731

line is drawn upon the forward zinc plate at the 500 links graduation, the readings of three standardised thermometers (suspended one at the centre, the others at one-sixth of the length of tape from each end) being recorded simultaneously. The operation being extended the full length of the base, the recorded measurements are corrected for the error, if any, of tape; for variation from standard temperature; for stretch due to excess or deficiency in pull as compared with that applied when the tape was standardised; for sag; for reduction from slope-measurements to the horizontal value; and for reduction to sea-level.

5. The base should be measured independently three times in calm and cloudy weather or at night. To counteract errors due to uncertainty of temperature of tape, measurements should be effected with both rising and falling temperatures, and the coefficient of expansion of the tapes used specially determined. The mean of the several corrected and reduced measurements is to be adopted. If possible it is desirable that the base be divided into two or three sections, and compared one with the other through subsidiary triangles.

Method of angular observation.

6. According to reliable authorities, the repeating system has again come into favour. The method of recording at each trigonometrical station the bearings of the surrounding stations of the polygon and also of a few of the more prominent outside ones in continuous rounds, all referred to a common origin, has, however, been found in this colony to be expeditious and sufficiently accurate to meet the standard of accuracy prescribed as necessary for the control of settlement surveys.

Sets of readings.

7. The observations at each station should comprise two complete sets of readings, taken as follows:—
   (a.) Face direct: Vernier A set approximately to back bearing of the station of origin; all other stations being observed in consecutive order from left to right, the return reading of origin being also booked, care being taken that no station is overshot.
   (b.) Face reversed: Vernier A being moved on 30° approximately, with proportional forward movement on vernier; this round to be taken in the same manner, except that the rotation is to be from right to left.
   (c.) Face direct: Vernier A set forward approximately another 30°, with proportional movement on vernier; this round to be taken from left to right.
   (d.) Face reversed: Vernier A being set forward approximately another 30°, with proportional movement on vernier; this round to be taken from right to left.

A 10 in. Everest with three verniers will thus furnish a complete set of twelve readings, the arithmetical means of which, corrected proportionately for the difference between the starting and return readings, and also reduced to the true back bearing of origin, will complete the set. Two sets taken under favourable conditions should suffice, but the observer will use his judgment as to the expediency of supplementing them. If the theodolite be provided with only two verniers, six rounds as above must be taken to constitute a set.

Hypsometrical observations.

8. The angles of elevation or depression to the surrounding signals, the exact heights of the telescopic axis and of the signal sighted to in each case being noted, should comprise not less than two sets of readings—one with circle right, the other with circle left. The divisions of the eye and object end of the level attached to the vertical arc in each case should be noted, in order that the vertical angles may be reduced to their true value.

Particulars to be recorded.

9. Each set of observations should be headed to show clearly the observing station, stations observed to, instrument used, the state of the weather, height of barometer and thermometer, observer’s name, and the date and hour. Notes should also be taken of the definition or otherwise of the signals—whether clear, sharp, and