Most of the investigators of the eighteenth century occupied themselves with studies indirectly or directly caused by the invention of the steam engine, that is with studies relating to heat and light; but, by reason of the interdependence of all the physical sciences, their investigations led them automatically into the allied fields of acoustics and electricity. Their investigations led even further; they led to the establishment, on the ruins of the illusions of alchemy, of a wholly new and supremely important science, chemistry. One of the most important inventions of a purely scientific character made during the period was one that has never been known by any other name than "Atwood's machine." It is an interesting illustration of the addition of invention to investigation, in that its end was--merely investigation; and it reminds us of a fact that many people are prone to forget, that invention may be applied to almost any purpose whatever, and that even a "machine" may be devoted to a purpose not utilitarian. Atwood's machine was the outcome of studies into the relations between force and a body to which force may be applied. Galileo had shown that a body subjected to a constant force, like that of gravity, will gradually acquire a velocity and at a constant rate; and also that this rate, or acceleration, is proportional to the force (leaving out the effect of air resistance). Atwood's machine consisted merely of an upright with a pulley at its upper end over which passed a cord, to both ends of which weights could be attached. In any given experiment, a weight was attached to one end and allowed to fall free; but another weight could automatically be attached to the other end by a simple device, when the first weight had fallen through any predetermined distance. If the added weight were equal to the first weight, the velocity of movement became uniform at once; while if it were less, the velocity approached uniformity to a degree depending on the approach to equality of the two weights. While this machine did not establish any new law, or prove anything that Newton had not proved before, it supplied a very valuable device for conducting quantitative experiments with actual weights, and for instructing students.