Page 2 Advertisements Column 3

Wellington Independent, Volume XXIV, Issue 2806, 6 March 1869, Page 2

both diagonals when the triangles arrange themselves in quadrilaterals, as is sometimes the case where the breadth of open country is limited. Stations to be marked with boxes or blocks of totara, and with an iron peg, and to have piles or mounds raised round them. 17. An excellent and also an economical method of marking the centre of the stations is with boxes made of the heart of totara or other durable wood, into which the pole of the signal flag is inserted. These should be let into the ground previous to commencing the observations for bearings. In distant parts of the country it will be sufficient to place a sawn totara block 2 feet long and 6 inches square. An iron peg of the usual Government pattern is also to be placed at the station in case of accident from fire. To render the stations more easily distinguishable a pile of stones or a mound of turf about five feet in diameter at the base, and five or six feet in height is raised round the station. Elimination of errors caused by false graduation on the limb of the Theodolite. 18. In order to diminish the effect caused by errors of graduation on the limb of the theodolite it is important that the bearings be read on the whole limb by successively shifting vernier A a certain number of degrees after every round of observations according to the number of repetitions required. Let a be the number of repetitions proposed, b the number of verniers, and x the shift of vernier A in degrees, then x=360°/ab. Four repetitions will generally be found sufficient, and as there are two verniers to the 8 inch theodolite by formula x = 360°/42 = 45°, or the number of degrees vernier A has to be shifted after every complete round of observations. If an Everest Theodolite having three verniers be used, and the same number of repetitions be required, then the shift on the limb would be 30°. Errors arising from dislevelment and want of collimation. 19. The theodolite is further liable to errors arising from dislevelment, and from want of correct collimation. However carefully the mechanical adjustments for these errors may be performed they can never be made perfect, and are constantly liable to disarrangement. But by simply reversing the instrument in altitude and azimuth the whole of the above errors become destroyed. Therefore observations cannot be considered satisfactory unless this system of reversion is attended to, taking care that the number of observations are equal upon each reversion of the instrument. Errors arising from false centring. 20. There is still another source of error arising from the centre of rotation of the instrument not being a fixed point. The reading off of all the verniers at every observation cancels this effect. Method of observing Bearings. 21. The instrument being firmly fixed over the centre of the station and carefully levelled is to be set with vernier A nearly on the back reading of a station from which observations have already been made, and the cross wire of the telescope brought to bisect exactly the signal of that station. Then un-clamp and slowly move the instrument in azimuth from left to right until it points nearly to the next station in order for observation, clamp, bisect, and read off as before. In this manner observe and read successively each station round the circle, until the station first set upon is read again, and record its return readings. Should these not differ materially from the former readings it is a proof that the instrument has not shifted in position during the round of observations which may now be considered as one set. Second set of bearings. 22. Next set the instrument with vernier A 45° in excess of the given bearing of the back station referred to before, unclamp the lower plate, and turn the whole body of the instrument round from right to left until the cross wire bisects the said station. Then proceed to re-observe every station as before directed. This will give a second set of observations on the same face of the instrument. Third set of bearings. Face reversed. 23. Now turn the instrument completely round in altitude, shift vernier A 45° further on the horizontal limb, then unclamp the lower plate, and turn the body of the instrument round in azimuth until the cross wire again bisects the reference station; the instrument by so doing will become completely reversed; the face of the vertical circle if before on the right hand side will now be found on the left of the observer. Re-observe all the stations for a third set of bearings to be termed "face reversed". Fourth set of bearings. Face reversed. 24. Lastly shift vernier A still further 45° on the horizontal limb, and observe a fourth set of bearings also "face reversed". Arithmetical means of the hearings to be taken. 25. By attending to the above directions, four separate observations, taken equally upon both faces of the instrument, and comprising eight readings (since there are two verniers) are obtained for each station, and the arithmetical mean of the readings is to be taken as the mean bearing, with every chance of its being within a very few seconds of the truth, providing that the