214. The _second_ having been obtained, a pendulum can be constructed which shall, when put in motion, perform one vibration in exactly one second, in the latitude of Greenwich.[35] If we were inventing measures, it would be convenient to call the length of this pendulum a yard, and make it the standard of all our measures of length. But as there is a yard already established, it will do equally well to tell the length of the pendulum in yards. It was found by commissioners appointed for the purpose, that this pendulum in London was 39·1393 inches, or about one yard, three inches, and ⁵/₃₆ of an inch. The following is the division of the yard. MEASURES OF LENGTH. The lowest measure is a barleycorn.[36] 3 _barleycorns_ are 1 _inch_ 1 in. 12 _inches_ 1 _foot_ 1 ft. 3 _feet_ 1 _yard_ 1 yd. 5½ _yards_ 1 _pole_ 1 po. 40 _poles_ or 220 _yards_ 1 _furlong_ 1 fur. 8 _furlongs_ or 1760 _yards_ 1 _mile_ 1 mi. Also 6 _feet_ 1 _fathom_ 1 fth. 69⅓ _miles_ 1 _degree_ 1 deg. or 1°. [33] The minute and second are often marked thus, 1′, 1″: but this notation is now almost entirely appropriated to the minute and second of _angular_ measure. [34] The measures in italics are those which it is most necessary that the student should learn by heart. [35] The lengths of the pendulums which will vibrate in one second are slightly different in different latitudes. Greenwich is chosen as the station of the Royal Observatory. We may add, that much doubt is now entertained as to the system of standards derived from nature being capable of that extreme accuracy which was once attributed to it. [36] The inch is said to have been originally obtained by putting together three grains of barley. A geographical mile is ¹/₆₀th of a degree, and three such miles are one nautical league. In the measurement of cloth or linen the following are also used: 2¼ inches are 1 nail 1 nl. 4 nails 1 quarter (of a yard) 1 qr. 3 quarters 1 Flemish ell 1 Fl. e. 5 quarters 1 English ell 1 E. e. 6 quarters 1 French ell 1 Fr. e. 215. MEASURES OF SURFACE, OR SUPERFICIES. All surfaces are measured by square inches, square feet, &c.; the square inch being a square whose side is an inch in length, and so on. The following measures may be deduced from the last, as will afterwards appear. 144 square inches are 1 square foot 1 sq. ft. 9 square feet 1 square yard 1 sq. yd. 30¼ square yards 1 square pole 1 sq. p. 40 square poles 1 rood 1 rd. 4 roods 1 acre 1 ac.