Surveying and Levelling Instruments, Theoretically and Practically Described. / For construction, qualities, selection, preservation, adjustments, and uses; with other apparatus and appliances used by civil engineers and surveyors in the field.

by William Ford Stanley

Fig. 120.—Vernier scale, reading 23° 12′.

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319.—In Fig. 120 the arrow upon the vernier scale is shown reading at a position beyond 23°, which we then know must be 23° n′. Now, if we look along the vernier, the lines of this and the scale appear coincident at the twelfth division of the vernier; consequently, the n′ is 12′, and the reading is altogether 23° 12′.

320.—Learning the reading of the vernier is very similar to that of the clock, wherein a child at first gets confused by the difference of value of the minute hand and the hour hand. In the case of the vernier we have only to get clearly in our minds that the degree reading and the vernier reading are quite distinct processes, in which the vernier reads minutes only, and this by coincidence of lines only, and that it has nothing to do with degrees, which are indicated by the arrow only. The arrow may be assumed to be placed on the vernier scale to save an unnecessary line of division; but this practically might just as well be placed quite outside of it, as it has nothing whatever to do with the vernier reading.

Fig. 121.—Vernier scale, reading, 23° 47′.

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321.—It is important to make this matter of reading the vernier clear; therefore in Fig. 121 the index arrow and vernier are shown reading past a half degree. At this position the arrow reads 23·30 on the limb + the vernier, or 23° 30′ + n′ of the vernier reading. We find the coincident line of the vernier with the limb is at 17, therefore the reading is 23° 30′ + 17′ or 23° 47′.

322.—The principle of the vernier, upon which it takes its reading from the coincidence of lines, as just stated, points out that the figuring of values of points of coincidence may be varied at discretion, and the zero index may be in any convenient position. The above described is the common reading to the theodolite and many other instruments. In mining dials and some other instruments the zero is placed in the centre. We may, for example, take a central reading with a vernier reading to 3′, wherein the circle being divided into degrees; the vernier is then, necessarily, in the direct method, divided into twenty divisions (20 × 3 = 60) which correspond with nineteen degree marks of the circle. With a central reading the vernier in this case is figured 30, 45, 0, 15, 30. This is rather a simple reading, as the zero to which an arrow is attached gives the true bearing, and it is readily seen to which degree it refers.