208.—Perhaps, upon the whole, the conditions which formerly rendered the Y-level undoubtedly the best practical level have so much changed that the more solid construction of the dumpy may entirely supersede it, as it seems likely to do in modern practice, and the optician will lose his ideal. Some reasons for this may be stated, but whether sufficient is a question. The manufacture of object-glasses of good figure and proper centring was formerly understood by a few scientific opticians, who were principally engaged upon astronomical telescopes, so that, with the exception of those made by Troughton and Simms, no very good and accurately centred lenses were used in surveying instruments. With bad centring alone, in ordinary telescopes, the webs in collimating were drifted aside, and needed the Y system of adjustment to make the telescope workable for levelling. In the modern good object-glass, of which there are now several makers, the centring is so nearly perfect that the webs in adjustment fall in the centre of the diaphragm when it is placed true to the cylindrical axis of the telescope. If the webs are placed as suggested without further adjustment, no very serious interference is caused by want of collimation of the axis. With this fact in view, the instrument maker needs leave little space for adjustment of the webs for centre displacement to become a source of error to persons not used to adjustments.
209.—Further, with a well-centred object-glass, as it leaves the hands of the scientific optician, and a solidly constructed adjustment to collimation being provided for in the making of a level, true working may be done even if there is a small error in the collimation. The late William Gravatt, C.E., was of opinion that firm construction, compact form, and plenty of light in the telescope were more important than easy facilities of adjustment. There is no doubt he found the less open adjustments the better in the hands of the imperfectly trained assistants who were pressed into service during the railway mania of 1848. At any rate, at this period we have his invention of the "Gravatt," or, as it was afterwards termed, the "Dumpy" level, which has remained with us with slight modifications in its mechanical parts and with increasing popularity until the present time. The late Mr. Troughton, recognising the same facts, also made a level in which there was no adjustment to the supports of the telescope after it left the hands of the maker. In his level he also left no adjustment to the bubble tube, which no doubt would prevent tampering, but which could scarcely be called an improvement; as this tube is liable at all times to be broken, therefore to need replacing with another tube, which cannot be made quite similar, and therefore needs easy means of adjustment for a surveyor to replace it when abroad. This level has gone out of use, but it is mentioned here, as the old engraving of it remains in some of our modern text-books.
Fig. 59.—Dumpy level.
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210.—The Dumpy Level.—One of the most important structural improvements made by the late William Gravatt in his dumpy level, was the addition of a cross bubble, shown end-view in Fig. 59 at CB. This improvement over the old form of Y-level permitted the setting-up of the instrument to be completed approximately, without turning the level a quarter revolution backwards and forwards several times during the operation, as was necessary in the setting-up of the Y-level. The compact form, lightness, and large field of view in the telescope otherwise commended it to civil engineers, when Gravatt had pointed out the possibility of sufficient practical adjustment without resorting to the cumbrous proportions of the Y-level as it was then made. Modern experience has shown that the dumpy form of telescope could very well be applied to the Y construction, and this has been done, as shown in the preceding pages; but at the time the dumpy was invented by Gravatt, the Y-levels were very commonly made 20 inches or more in length of telescope, and were altogether very flimsy affairs. Gravatt's 12-inch level was found to be quite equal in power and of less than half the bulk and weight. A 12-inch dumpy should read the ·01 foot on a Sopwith staff, which is described in the next chapter, at 5 chains with a webbed or glass diaphragm, Fig. 61; with a more open reading than Sopwith's staff a greater distance than this. A 14-inch dumpy should read the ·01 foot at 10 chains.
211.—The Dumpy Level of modern form is represented in the engraving, Fig. 59. It consists of a telescope, fully described art. 94, which carries a ray shade RS at the object-glass end, to work in the field to eastward or westward facing a low sun. The eye-piece EP is adjustable to the webs in the telescope by pressure in or out. Two straps or bands are accurately fitted and soldered round the tube of the telescope; one of these carries a hinge joint, and the other a pair of locking nuts to support the level tube GG, and at the same time permit its adjustment. The level casing tube has two three-quarter bands, which slide upon it, pointed at one end GG: these adjust to the length of the bubble for changes by temperature. The lower part of each strap-piece is left a solid block of metal, to give very firm support to the telescope as it rests upon the limb L beneath. The limb may be either a casting with a socket screw only in its centre, or a compass box may be formed in the centre and the socket screw be placed under this, as it is shown in the figure at S. The attachment of the telescope support to the limb is made by three screws, two of which draw the limb down, and one in the centre presses it upwards, as shown in the section Fig. 60—CC′ telescope, TT′ drawing screws, P pressing screw.
Fig. 60.—Attachment of telescope block to limbs.
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