The practical use and working of a vernier is very simple. It is applicable to all measurements where minute accuracy is required, whether the quantity to be measured is itself a length, as the reading of a barometer, or is determined by the distance passed over by a moving point relatively to a uniformly divided scale straight or curved, as in the angular readings of an astronomical instrument. In its simplest form too, the principle on which it depends is very obvious. Supposing, for instance, we require to read a scale to the one hundredth of an inch. To graduate such a scale directly would require a large number of minute subdivisions, which would be troublesome to make with the requisite accuracy and very confusing to the eye in reading.