measures of capacity, by taking the weight of cubic millimètres, centimètres, decimètres, or mètres of water at their maximum density, that is, at 4° C. or 39° Fah. Unit of time or duration.-The unit of time or duration is the same for all civilized countries. The twenty-fourth part of a mean solar day is called an hour, which contains sixty minutes, which again is divided into sixty seconds. The second is universally used as the unit of duration. Another unit of time is the period occupied by the earth in making one revolution around the sun, in reference to an assumed fixed star, which unit is called a sidereal year, and contains 365 days, 6 hours, 9 minutes, 9.6 seconds mean solar time. Unit of velocity. The units of velocity adopted by different scientific writers vary somewhat; the most usual, perhaps, in regard to sound, falling bodies, projectiles, etc., is the velocity of feet or mètres per second. In the case of light and electricity, miles and kilomètres per second are employed. Unit of work. In this country and in England, the unit of work is usually the foot-pound, viz., the force necessary to raise one pound weight one foot above the earth in opposition to the force of gravity. A horse-power is equal to 33,000 pounds raised to a height of one foot in one minute of time. In France the kilogrammètre is the unit of work, and is the force necessary to raise one kilogramme to a height of one mètre against the force of gravity. One kilogrammètre 7-233 foot-pounds. The cheval-pouvoir is nearly equal to the English horse-power, and is equivalent to 32,500 pounds raised to a height of one foot in one minute of time. The force competent to produce a velocity of one mètre in one second, in a mass of one gramme, is sometimes adopted as a unit of force. Unit of pressure. The pressure of the atmosphere at the level of the ocean, with the barometer at 30 inches, is taken as the unit in estimating and comparing pressures and elastic forces. TABLE SHOWING ALL THE UNITS OF LENGTH RECOGNIZED IN ENGLAND SINCE THE SIXTEENTH CENTURY. (6.1538 inches side of cube of wine gal. of 223 cub. in.) (12.907 inches side of cubic bush. or 2150 42 cub. in.) (38.73 inches side of cubic wine ton of 58212 cub. in.) (5.0397 feet side of cubic cord or 128 cubic feet.) 1.25 yards (5 quarters) 1-644 ell English (6 quarters) 2.59 geographical miles (3 statute miles) 1 league. A cubit is two feet. A great cubit is eleven feet. A palm is three inches. A span is ten and seven-eighth inches. A pace is three feet. A barrel of flour weighs 196 pounds. A barrel of powder weighs twenty-five pounds. Atoms and Molecules. The term atom has been exclusively appropriated by the chemist, while the mathematician and physicist have preferred to adopt the word molecule to signify those ultimate constituents of matter upon whose motions and relations depend the various states of all bodies solid, liquid, and gaseous. It is said that atoms are attracted to each other by the attraction of cohesion, and repelled by the force of repulsion. By the action of both these forces, the atoms are kept in a state of rest. The solidity of a solid depends upon the fact that each pair of atoms is in this state of equilibrium. These atoms are supposed to be of an oblate, spheroidal form. The word particle is also freely made use of as involving no hypothesis, and meaning simply a small part of any body. Molecule has been defined by Maxwell as "the smallest possible portion of a particular substance;" and, again, as "that small portion of the substance which moves as one lump in the motion of agitation." Every substance is now supposed to be composed of an immense number of molecules, which, even in the solid state, are never entirely at rest, and in the gaseous are in a state of perpetual violent commotion, rushing about in straight lines in all directions with inconceivable rapidity. The difficulty of proving or disproving the molecular theory lies in our inability to determine the size or shape of a molecule by any means in our power. The most powerful microscope fails utterly to show them, and should some material for lenses be discovered infinitely superior to glass or other material at present in use, we should fall far short of appreciating a molecule through the vision. |