In Fig. 27, zero on the vernier is exactly in line with 29 inches and 5/10ths of the fixed scale; the reading, therefore, is 29·500 inches. The vernier line _a_ falls short of a division of the scale by ·002-inch; _b_, by ·004; _c_, by ·006; _d_, by ·008; and the succeeding line by ·010. If the vernier be adjusted to make _a_ coincide with _z_ on the scale, it will have moved through ·002-inch; and if 1 on the vernier be moved to coincide with _y_ on the scale, the space measured will be ·010-inch. Consequently, the figures 1, 2, 3, 4, 5, on the vernier, measure 100ths, and the intermediate lines even 1000ths of an inch. In Fig. 28 the zero of the vernier is between 29·65 and 29·70 on the scale. Glancing up the vernier and scale, the second line above 3 will be found in a direct line with one on the scale; this gives ·03 and ·004 to add to 29·65, so that the actual reading is 29·684. In those instances where no line on the vernier is found _precisely_ to coincide with a line on the scale, and doubt arises as to which to select from two equally coincident lines, the rule is to take the intermediate 1000th of an inch. [Illustration: 27. and 28. The Vernier.] For household and marine barometers such minute subdivisions of the scale are unnecessary, and the scales of such instruments are therefore divided only to 10ths, and the verniers made only to read to 100ths of an inch, which is effected by making the vernier 9/10ths or 11/10ths of an inch long, and dividing it into 10 equal parts. In “taking a reading” it is important that it should be done as quickly as possible, as the heat from the body and the hand is sufficient to interfere with that accuracy which is necessary where the intention is to compare the readings with those made by other observers. This facility is soon acquired by a little practice. [Illustration: 29. Farmer’s Barometer. Scale about 1/7.]