Fig. 99.—Sopwith's staff.
Fig. 100.—Rogers Field's staff.
Fig. 101.—Col. Strange's staff.
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263.—The original form of reading designed by Sopwith is still much more in use than any other. It is similar in pattern to Fig. 96, shown in detail for 1 foot Fig. 99. The dots at the end of the lines shown in the figure were introduced by the author to render this staff more distinct than when lines only are used, as in the ordinary pattern.
264.—Sopwith's pattern is sometimes printed on paper for pasting on the staff, and in this manner the staff comes out much cheaper than by drawing the readings in solid paint. Paint, however, is strongly recommended, not only because it wears much better and keeps cleaner, but that the painting and varnishing add very much to the durability of the staff, exposed as it must necessarily be to rainy weather; further, the paper, however well it is fixed at first, is liable to creep away from the edges of the staff, and leave a space into which rain enters very freely by capillary attraction; but it does not again freely evaporate, so that it rots the staff and makes the paper reading after a time mouldy. It is, nevertheless, convenient to take a set of first length papers if a surveyor is going abroad, as from accidents—grazing by carrying the staff with the tripod of the level, etc.—the first length of surface is very liable to become too much injured and effaced for fair reading. A description of fixing the paper will be given further on.
265.—For Reading the Sopwith Staff, the foot readings are taken from the tops of the red figures. The ·1 foot figures are in black, and are all odd numbers, 1, 3, 5, 7, 9. These read also from the top. The height of the figure is exactly ·1 foot, so that the bottom of each figure reads the lower even number—thus the bottom of 3 reads 2, of 5 reads 4, of 7 reads 6, and of 9 reads 8. The 6 and 9 foot figures if made alike, from effect of telescopic inversion, may cause error. The author has for many years made the head of the 9 a solid black block to avoid this.
266.—Various Readings.—A very large number of surveyors design their own staff readings. This was formerly very much the fashion, consequently a great number of patterns come before the manufacturer. The author for about twelve years kept a copy of what he considered the most meritorious of these patterns, both for future reference and to judge of their comparative merits. This was discontinued, as it was found that the number of designs became a little perplexing, and they were rather dangerous to show to a customer, who often selected from its appearance a pattern which proved afterwards unsatisfactory in use.
267.—Rogers Field's and Colonel Strange's Readings.—The author made some experiments to obtain a clear staff, readable beyond the ordinary range of staves with a 14-inch level; but much more complete experiments were made with the author's set of patterns by Mr. Rogers Field, C.E., whose ingenuity is well known. This gentleman finally designed a staff which in the author's opinion is still one of the best, but it has not generally pleased the profession: this is illustrated, Figs. 97, 100. The author has tried it at all distances: at 20 chains he has found a reading of ·01 foot could be taken approximately with a good 14-inch level with his point index-stop level, [Fig. 75]. The late Colonel Strange made a series of experiments with the author's patterns placed at 10 and 20 chains distance. He also had for these experiments one of Mr. Rogers Field's staves. He arrived at the conclusion, for distant reading particularly, that the black markings on all the twenty staff patterns he had were excessively heavy, so that the lightest and most open readings were the clearest. This led him to design a staff, a part of which is shown in Fig. 101, which has been since generally used on the great India survey. This staff somewhat resembles the English ordnance pattern. The fault found with these patterns is that they do not read the ·01 foot, which is necessary for close reading in hilly districts, otherwise they may be read very clearly at a distance of 20 chains, where the Sopwith becomes a blur. We may take it that the surveyor, if he be a fairly good draughtsman, would subdivide the ·05 block to the ·01 foot; but it is argued that his assistant, who might be a fair leveller, might not. Another objection is that the reading is on one side and is not cut through by the horizontal web, so that a white margin can be seen in the telescope on both sides of the vertical webs, between which it is most pleasant and exact that the reading should be taken. This objection does not, however, hold for the point reading, [Fig. 75]. Colonel Strange's pattern has not been very generally accepted by civil engineers. The author tried to meet the matter by making the block ·05 foot, but so subdivided as to indicate ·01 foot. This has frequently been preferred to his dotted Sopwith.
