Experience has shown that the measurement of all physical quantities may be expressed in terms of three fundamental magnitudes. Those commonly chosen for this purpose are _time_, _length and mass_ _or quantity of matter_. It may be assumed that our ideas of time and space are sufficiently exact for all practical purposes. The subject of matter, however, requires more particular consideration. Of the three magnitudes named, matter alone is directly cognizable by the senses, and invested with a variety of interesting properties. For present purposes matter may be defined as anything that can be weighed, and the quantity of matter as proportional to its weight; _i. e._, its attraction towards the earth. The _weight_ of a body is the force it exerts in consequence of its gravity, and is measured by its mechanical effects, such as bending a spring. We weigh a body by ascertaining the force required _to hold it up_, or to keep it from descending. Hence, weights are nothing more than _measures of the force of gravity_ in different bodies. Again, Gravitation, the most feeble of physical actions between small masses, is almost imperceptible; yet it is an energy abundant in proportion to the quantity of matter in the universe, and fully competent, by its gradual condensing agency, to account for the origination of planetary systems and their movements. It is not strange, therefore, that by some physicists this energy is supposed to be the beginning of that of which all other forms of force are residues or metamorphoses. Gravity is the name especially given to its terrestrial manifestations. _A particle or body without a sphere or spheroid, solid or hollow, is attracted to the center of the mass of such body; within a hollow sphere, it will remain at rest at any point._ At different depths below the earth’s surface, a body will be attracted with a force diminishing as the distance from its center decreases. The slight variation in the gravitating force of the same falling body at different heights is in practice usually disregarded. The weight of a body, as the measure of its gravitating tendency, must vary both with mass and with the force acting on it; hence, from the form of the earth, the same body at the sea level _will weigh less and less as it is removed from either pole toward the equator_. An elevation above the sea level gives a like result. A stone falls through a less distance in a given time on a mountain than in the valley below, less at the equator than at either pole. The loss of weight in these cases cannot be tested by lever scales, in which this loss is equal on both sides; but it may be by the spring balance, in which bodies are weighed by the pull they exert against the elasticity of a coiled wire. The effect of centrifugal force, increasing from the pole to the equator, co-operates with increasing removal from the earth’s center to lessen weight; the result of the