The heart we regard as the grand regulator of the blood's flow; and it is admirably situated for measuring out a regular portion of blood at every contraction. John Bell, believing in the Harveian theory, said, "It is awful to think of the unfixed position of the heart;" and Dr. Arnott declared that "the heart, the heart alone, is the ragged anomaly in the laws of fitness in mechanics." The heart was now seen to have a right position; for it should swing loose that its moorings be not endangered; and, as whatever impugns the Creator's unerring wisdom must be wrong, so the presumption is, that whatever vindicates it must be right. My hypothesis assumed the principle, that, if an endless hollow tube be filled with a liquid, the liquid can be made to circulate perpetually, if it be heated at one point and cooled before its return. A drawing of the simple apparatus by which this problem was proved, is given in my published work on "the Motive Powers, &c." The figure which represents this apparatus gives the learner the most simple idea possible of the connection of the respiratory and circulatory systems, and of the combination of the two motive powers; the first, or chemical, coming from the lungs, and the second, or mechanical, from the heart. Suppose the heart divided into right and left hearts by dissection at the septum: the circulatory system might then be represented by an endless tube. Let such an one, nine or ten feet in length, and of one inch bore (to be filled with water) be placed upon a horizontal table. Let an enlargement of the tube be made by a tin vessel to represent the lungs, which shall contain about one-fifth part of the water. Let the tube connected with the right side of the vessel have, at a little distance from the vessel, a smaller enlargement, composed of india-rubber, which can be grasped by the hand, to represent the heart's right ventricle, with a valve on each side opening towards the tin vessel, the two to represent the tricuspid and semi-lunar valves. Let the whole be made nearly full of water; then, under the tin vessel (representing the lungs), let a fire be made. As the water heats, it will expand; and as the valve closes to the right, it will go off to the left side of the vessel. But, as no water will come in from the right, on account of the valves, there will be no current. Now let the hand grasp the india-rubber, and the fluid between the valves being displaced by its pressure, all the water will go towards the tin vessel, because, while the valve representing the tricuspid would close, that representing the semi-lunar (between the mimic heart and lungs) would open--and very freely; because the expansion made by the heat under the tin vessel had created a vacuum, and thus made a suction power to draw it forward, while there is a driving power behind to force it onward into the tin vessel. Then relaxing the hand, a vacuum will exist between