PART SIXTH. VARIOUS MATTERS BEARING ON THE MAIN SUBJECT. Section One: Notes on Divers Things. 387. Nature and Object of Part Sixth.-The controlling idea throughout this Treatise was to give with due fullness whatever is intimately connected with the Compass and its Deviations, and to treat subsidiary matters only to such extent as would conduce to an intelligent knowledge of the main subject; but many things bear more or less upon this: to deal with these with the detail they require in order to be well understood, would extend the work far beyond its proper sphere; and it has occurred to the writer that, for those who may wish to pursue such parts further than they are given here, it would be useful to have reference to works that have been found clear and explicit upon the different subjects. This Part, therefore, contains reference to such books as will give the information indicated by the headlines of Articles 388 to 400. 388. Constants of instruments and of formulas: matters referred to in Part First for explanation here.-An absolute instrument is one whose scales give values in C.G.S. units; a small instrument, portable and convenient for work, generally has an arbitrary scale: the same quantity having been measured several times by a small instrument and an absolute one, affords one of the Constants of the small one, and the other Constants are found in a similar way. Those quantities in an equation which retain a fixed value are the Constants, and such, for example, are the Magnetic Coefficients in the deviation formula-the Course is the variable. For determining the constants of formulas, see Laboratory Manual of Physics and Applied Electricity, by E. L. Nichols, vol. i, page 13; and Physical Measurements, by F. Kohlrausch, page 11. IN PART FIRST, reference was made to PART SIXTH for explanation of certain matters as follows: Page 257-Variation of gravity with latitude: see Mechanics, by Aug. W. Smith, page 179; and Traité de Physique, par P. A. Daguin, tome i, page 126. Pages 258 and 260-The turning moment of two magnets: this will be found in Art. 399, below. Pages 293 and 296-formulas: these, also, are referred to in Art. 399. Page 514-equation (1): deduced in Daguin's Physics, vol. i, page 533. 389. Scales: Earth's Curvature: Duration of light flashes. For converting linear scale divisions into angular measure, see Kohlrausch, page 116. The curvature of the Earth is about eight inches to the mile. An impression of light takes about one-seventh of a second to be conveyed by the optic nerve: a red-hot stick whirling during this length of time leaves the impression of a red circle, and for a longer time only a portion of a circle. The portion of time the flash lasts, indicates the portion of the impression made: it is the principle of the moving pictures. 390. Materials absorbent of Moisture, of Heat, and of Light.—Chloride of calcium, or pumice stone, saturated with strong sulphuric acid, will absorb moisture. A solution of alum intercepts heat waves but allows those of light to pass; while a solution of iodine will intercept light waves, but allow those of heat to pass. A thin slab of pure rock salt allows heat waves from every source to pass freely through it, as transparent glass does white light. Section Two: Units and Standards of Measure. 391. Fundamental and Derived Units.-All measurements depend upon Length, Mass, and Time, and these quantities are therefore called FUNDAMENTAL: upon them are based measures of Area, Volume, Density, Force, Velocity, Acceleration, Work, Energy, and Momentum, which are hence said to be DERIVED. Nations differ as to the specific length and mass they establish for standard units: the English have the foot-grain-second system, and the French the centimeter-gramme-second (C.G.S.) system; this is also the system of many other nations in the daily dealings of life, and it is almost exclusively that of scientific investigation. and literature the world over. Materially, the unit of length-the centimeter is the onehundredth part of the distance at o° Centigrade between two parallel lines engraved on a certain bar of platinum-iridium alloy, deposited in a vault in the Laboratory of the Comité Internationale des Poids et Mesures at Sèvres, near Paris. This bar is the Mètre Prototype. The gramme is the one-thousandth part of a platinumiridium piece known as the Kilogramme Prototype, deposited as above. (Webster, Theory of Elec. and Mag.) Regarding the C.G.S. system, see: Units and Physical Constants-J. D. Everett; Absolute Measurements-Andrew Gray, vol. i, chapter iii, and vol. ii, part ii, chapter ix; Electricity and Magnetism-Silvanus Thompson, page 263; Electricity and Magnetism-J. E. H. Gordon, vol. i, page 47; Theory of PhysicsJ. S. Ames, pages 7, 75, and 101; and Electricity and Magnetism— J. C. Maxwell, vol. i, page 3. 392. Electrical and Magnetic Units.-There are two systems of these units-one based on the force exerted between two quantities of electricity, and this set are called electrostatic units: the other based on the force between two magnet-poles, and these are termed electromagnetic units. There is a most important link connecting them, which is explained on pages 521 to 543 of this Treatise. Both systems are derived from the fundamental quantities Length, Mass, Time, and become specific by using for these the C.G.S. units: this, however, introduces magnitudes of inconvenient size some too small, others too large-so that certain decimal multiples and submultiples of the basic absolute units have been agreed upon for practical purposes, and these have received the names of Volt, Ampere, Ohm, Coulomb, Farad, Watt, Joule, and Henry. The subject is treated in the following works: Elec. and Mag., Sil. Thompson, pages 130, 192, 265, and 344; Ames' Physics, pages 273, 299, 363, and 367; Gordon's Elec. and Mag., vol. i, page 257, and vol. ii, page 213; Gray's Abs. Meas., vol. i, chapter iii, and vol. ii, part ii, chapter ix; Everett-Physical Units; Principles of Dynamo-Electric Machines-Carl Hering. 393. Various kinds of angular measure. These are such as are reckoned in degrees; or in an arc equal to the radius; or by trigonometrical functions; or by solid angles: except the last, they are explained in Art. 296, and all are briefly dealt with by |