Fig. 430.—Thacher's slide rule.
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Although the co-ordinate spiral rule, as all varieties of slide rules, is based primarily upon the theory of logarithms, a knowledge of that theory is by no means essential to its practical use.
887.—Thacher's Slide Rule.—Fig. 430. This contains a shorter scale than Professor Fuller's, and the system is not quite so simple. Full printed instructions are given in the book supplied by the inventor, Mr. Edwin Thacher, of Pittsburg, U.S.A., or of the author, who is his agent for this country. The original divisions of this rule were made by the author. The scale is manufactured in the United States. There appears to be found some difficulty in its construction to keep the scales to true length and get them to exact position.
888.—Pocket Sets of Chain Scales.—These are made 3, 4, 5, and 6 inch. Three of 6 inch form the ordinary set. The chain scales, if three only, are 10, 20, 30, 40, 50, and 60; if six they generally contain the same scales with feet equal to the links. An extra scale with the ordnance or other scale of the country is found also useful for measuring from maps or plans. Some civil engineers prefer the pocket scales made wide with quite square ends, to be used as offsets or for sketching. These scales are generally made in ivory and placed in a light morocco or Russia leather case. The numbers of divisions of the scales should be stamped on the ends to prevent the wrong scale being drawn from the case.
Fig. 431.—Biram's anemometer.
Fig. 432.—Lowne's anemometer.
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