Let us now suppose two pipes, AB, BC, to be joined together as in Fig. 42, and to be separated by a fixed partition at B; and let a spiral spring be fixed in each. Let the spring AB be now pushed to the end A, while the spring BC is pushed to C, as in No. 1, and back again, as in No. 2, but always in opposite directions; then it is obvious that the partition B is in No. 1 drawn in opposite directions towards A and towards C, and always with forces equal to each other: that is, when B is drawn slightly towards A, which it is at the beginning of the motion, it is also drawn slightly towards C; and when it is drawn forcibly towards A, as it is at the end of the motion of the spring, it is also drawn forcibly towards C. If the partition B, therefore, is moveable, it will still remain fixed during the opposite excursions of the spiral springs; nay, if we remove the partition, and hook the end of one spiral spring to the end of the other, the node or point of junction will remain stationary during the movements of the springs, because at every instant that point is drawn by equal and opposite forces. If _three_, _four_, or _five_ spiral springs are joined in a similar manner, we may conceive them all vibrating between their nodes in the same manner. Upon the very same principles we may conceive a long column of air without partitions dividing itself into two, three, or four smaller columns, each of which will vibrate between its nodes in the same manner as the spiral spring. At the middle point of each small vibrating column, the air will be of its natural density, like that of the atmosphere; while at the nodes B, &c. it will be in a state of condensation and rarefaction alternately.