[Illustration: FIG. 232.--Relative positions of sun, moon, and earth at "spring" tides.] [Illustration: FIG. 233.--Relative positions of sun, moon, and earth at "neap" tides.] NEAP TIDES AND SPRING TIDES. The sun, as well as the moon, attracts the ocean, but with less power, owing to its being so much further away. At certain periods of the month, sun, earth, and moon are all in line. Sun and moon then pull together, and we get the highest, or _spring_ tides (Fig. 232). When sun and moon pull at right angles to one another--namely, at the first and third quarters--the excrescence caused by the moon is flattened (Fig. 233), and we get the lowest, or _neap_ tides. [39] In both Figs. 207 and 208 the degree of expansion is very greatly exaggerated. [40] As the sun passes the meridian (twelve o'clock, noon) the chronometer's reading is taken, and the longitude, or distance east or west of Greenwich, is reckoned by the difference in time between local noon and that of the chronometer. [41] For much of the information given here about clocks and watches the author is indebted to "The History of Watches," by Mr. J.F. Kendal. [42] We shall here notice only those gears which are included in the hub of the driving-wheel. [43] The original position of the moon is indicated by the dotted circle. INDEX. NOTE.--Figures in italics signify that an illustration of the thing referred to appears on the page. Aberration, spherical, of lens, 243. Acoustics, 294. Achromatic lens, 243. Action carriage of piano, 283. Advancing the spark, 102. Air-gun, _342_. Air-pump for cycle tyres, _340_; for Westinghouse brake, 199. Alternating currents, 164; dynamo, 164. Amperage, 125. Angle of advance, 57, 58; incidence, 268; reflection, 268. Aorta, 360. Arc lamp, 182. Archimedes, 412. Armature, 162. Arteries, 358. Arterial blood, 359. Atmospheric pressure, 350. Auditory nerve, 272. Automatic brakes, 188; signalling, 228; stoker, 399. Backfall, 298. Balance-wheel, 419. Ball cock, 366, _367_. Balloon, fire, 323; gas, 347. Barometer, aneroid, 328, _329_; and weather, 331; Fortin's, _326_; meaning of, 325; simple, _328_; wheel, _327_. Beau de Rochas, 89. Bell, diving, _332_; electric, 119, _120_. Bellows of organ, 303. Bioscope, 266. Blades, turbine, _81_, 83. Block system, 201, 212. Blood, arterial, 359; circulation of, _356_, _357_, 360; venous, 359. Blower-plate, 393, _394_. Boat, sails of, 346. Boiler, Babcock and Wilcox, _21_, 22; explosions, 34, 391; fire-tube, 21; fittings, 31; Lancashire, 25, _26_; locomotive, _20_, 23; multitubular, 21; principle of, 15; stored energy in, 32; vertical, _25_; water supply to, 39; water-tube, 21.