Gravity
Gravity names 3 different subjects across the 6 encyclopedias on this shelf. Each is set out below. 1 of them is described by more than one encyclopedia.
Gravity SPECIFIC, in physics
Collier's New Encyclopedia (1921)
SPECIFIC, in physics, the relative density of a substance; the weight of a body compared with that of another body having the same magniweighed tude. To obtain this, it is first in air, which shows its absolute weight. Next it is weighed in water, to show how much it loses in this element. There absolute have now been ascertained the weights of two bodies of equal bulk-viz., the one experimented on, and water, and also the ratio of these weights is that of their specific gravities. Let 1 be the weight of water, and first let the body that liquid, then the be heavier than weight which it loses in water is to the absolute weight as 1 to the specific grav- If lighter than water, ity required. as the then weight of the body in air, it sink plus the weight needful to make in water, is to its weight in air, so is 1 to the specific gravity. On this principle are constructed such instruments as Nicholson's portable balance. In solids and liquids the standard is generally disfor the gases, atmospheric tilled water; air. Specific gravity is proportionate to words may be used aldensity, and the most interchangeably.
Gravity See SPECIFIC GRAVITY
The Encyclopedia of Founding (1892)
See SPECIFIC GRAVITY. Gray Pig-iron is all pig-iron that contains a large proportion of its carbon in a graphitic state. Such irons may be distinguished by their crystalline fracture and dark-gray color. See CAST IRON. Green-sand is moulding-sand in a moist condition, and suitably mixed to form moulds in which the metal can be poured at once, without subsequent drying. See FACING-SAND; DRY-SAND MOULDING; GREEN-SAND MOULDING; DAMPNESS.
Gravity The tendency which a body has towards the center
Described by 2 of the 6 encyclopedias.
American Watchmaker and Jeweler (1892)
The tendency which a body has towards the center of the earth.
Collier's New Encyclopedia (1921)
CENTER OF. See CENTER OF GRAVITY, under CENTER.
What Has Changed in the Last Century
When the American Watchmaker and Jeweler defined gravity in 1892 as simply "the tendency which a body has towards the center of the earth," Newton's account had stood unchallenged for two centuries. Einstein's general theory of relativity, published in 1915, recast gravity not as a pull but as the curvature of spacetime by mass and energy — a view confirmed by the bending of starlight measured at the solar eclipse of 29 May 1919, two years before Collier's went to press without mentioning it. Gravitational waves, predicted by that theory, were first detected directly by the LIGO instruments in September 2015, and the first image of a black hole's shadow was published in 2019. Collier's account of specific gravity, by contrast, needs no correction at all: the method of weighing a body in air and then in water is unchanged, though the quantity is now usually called relative density.
Written for this edition, 2026.