Page:Popular Astronomy - Airy - 1881.djvu/123

Rh that the tension or pull produced by this one weight F, acting at the place E, does really support two tensions in different directions acting at the same point, namely, the tension produced by the weight C acting in the direction EA, and the tension produced by the weight D acting in the direction EB. Thus we may correctly say, that one pull in the direction EF does exert two pulls in different directions, AE and BE, for it really does keep the two cords strained to such a degree as to support the two other weights. We may say, on the other hand, that these two outside weights C and D support the middle one. The three pulls of the cords keep the point E in equilibrium; but they will support it only in one determinate position, according to the amount of weight which is hung to each cord. If I put another weight upon C, the position of the point E and the direction of the cords will immediately change; showing that for one proportion of the weights or tensions, there is only one set of angles between the different directions in which the tensions will keep the point E at rest; and, conversely, one set of angles of inclination requires the tensions to be in one certain proportion, in order that E may be kept at rest. Regarding the action of the two tensions in the directions EA, EB, as supporting the one tension in the direction EF, this may be considered as an instance of the combination of forces; and regarding the one tension in the direction EF as supporting two in the directions AE, EB, this may be considered as an instance of the resolution of forces, the one force in the direction EF being resolved into two forces in the directions AE, BE, and producing in all respects the same effects as two forces in the directions AE, BE. It may seem strange that a force acting in one direction can