Momentum
2 of the 6 encyclopedias on this shelf carry an entry for Momentum. Both are reproduced below, so you can see where they agree and where they differ.
American Watchmaker and Jeweler (1892)
The amount of motion in a body, which is obtained by multiplying its mass by its velocity. MOSELEY, CHARLES S. Mr. Moseley has been intimately connected with nearly every watch company in the United States, and as a mechanical engineer and designer of watch machinery, of the automatic type, he had no superiors and but few equals. Among those that have acquired a world-wide reputation may be mentioned the interchangeable stem-wind mechanism of the Elgin National Watch Company; the dust-band used by the same company, the best and cheapest ever made; the triangular hairspring stud; a patent regulator and the split chuck, an accessory now become universal and indispensable to every watchmaker in the land. He was born at Westfield, Mass., Feb. 28 1838. His first connection with watchmaking was in 1852, when he entered the employ of Dennison, Howard & Davis, at Roxbury, Mass. He followed the factory to Waltham and was employed in the capacity of foreman of the machine shop and later as master mechanic. In 1859 he became master mechanic of the Nashua Watch factory, and designed and built the machinery with which that watch was manufactured. In 1864 he joined the Elgin National Watch Company, then just starting, and was made general superintendent, in which capacity he remained with the company until 1877. MOTEL, H. A French chronometer maker, pupil and successor of Louis Berthoud. His chronometers were remarkable for their close rates and for their beautiful construction. He died in 1859.
Collier's New Encyclopedia (1921)
in mechanics, the force possessed by matter in motion; the product of the mass by the velocity of a body. Thus a ball of four pounds' weight moving uniformly at the rate of 18 feet in a second would have double the momentum that one of three pounds' weight moving at the rate of 12 feet per second would possess, for 4×18 is 72, and 3×12=36, or half as much. The force of percussion, that is, the force with which a moving body strikes an object, is the same in amount as the momentum of the former.
What Has Changed in the Last Century
The arithmetic in Collier's 1921 entry is still exactly right: mass times velocity, so a 4 lb ball at 18 ft/s carries twice the momentum of a 3 lb ball at 12 ft/s. What has gone is the loose language of calling momentum "the force possessed by matter in motion" — modern teaching keeps force and momentum strictly apart, force being the rate at which momentum changes. Two things the accounts could not include: Einstein's 1905 relativity makes the true momentum mv/√(1−v²/c²), a correction that only matters near the speed of light, and physics now assigns momentum to things with no mass at all, including light, which is how solar sails and radiation pressure work. Louis de Broglie's 1924 proposal linking momentum to wavelength, and Heisenberg's 1927 uncertainty principle pairing momentum with position, arrived just after Collier's went to press.
Written for this edition, 2026.