502.—Henderson's Dial.—This is an improvement upon an old form of circumferentor,[19] in which four sights are centred in opposite pairs so as to revolve about the vertical axis, so that one pair of sights may take any angle to the other pair. In Mr. J. Henderson's dial the improvement consists in making the compass larger, the needle being made to read by a vernier placed upon one end to 3′ of arc. Mr. Henderson prefers plain slit sights instead of slit and window sights, as before stated, which avoids the accidental derangement of the horse-hair.[20] The instrument combines some of the parts of Lean's dial, to be next described. Illustration of this instrument is given in Mr. B. H. Brough's Mine Surveying.
503.—Lean's Dial.—The inventor of this instrument was Mr. Joel Lean, a Cornish mine manager, who was well known at the end of the 18th century for his important improvements in mining apparatus. This dial is still popular in Cornwall and other mineral districts. In general construction the sights and limb on which they are mounted are the same as in the plain dial just described, [art. 491]. The legs are also
the same—other parts are additional or modified. In the engraving, Fig. 206, the sights and vertical arc with its telescope are shown mounted together on the limb. This is done to show the relative position of these parts: they could not in practice be used simultaneously upon the instrument. They are separately attached to the limb by the same pair of milled headed screws. As a general rule the telescopic arrangement, which will be described further on, is used above ground and the sight arrangement below. The details of construction are as follows:—
Fig. 206.—Mining circumferentor or Lean's dial.
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504.—The Tripod—of the mining circumferentor, in common with many other forms of dial, has the legs fitted directly between book-pieces, which are fixed to the lower parallel plate, as shown Fig. 206, thus dispensing with the separate tripod head, common to levels and theodolites. Otherwise the parallel plates are similar to those described for levels and theodolites, [art. 193], and are used in the same manner. The upper parallel plate in this dial carries the male axis, which fits into a socket attached below the centre of the limb in the manner just described for plain dial. The tripod stand, with its parallel plates attached, is generally packed in a pinewood case when out of use. The reason for attaching the legs directly to the lower parallel plate instead of having a tripod head is that it saves the extra elevation of the instrument by the depth of one screw fitting. At the same time it must be observed that it exposes the axis to the air by separating the instrument at this part when it is put by, which renders the axis difficult to be kept lubricated and in smooth working order. On the Continent and in America it is general to detach the legs only, on a plan shown, [Fig. 85], p. 140. This keeps the axis attached, and is probably the better plan, although it may be found a little more troublesome to erect the instrument.
Fig. 207.—Section of compass-box and axis of Lean's dial.