The tide-gauge, as shown in the illustration, consists of a cylinder, _A_, which is made to revolve on its axis once in twenty-four hours by the action of the clock, _B_. A chain, to which is attached the float, _D_, passes over the wheel, _C_, and on the axis of this wheel, _C_ (in about the middle of it) is a small toothed wheel, placed so as to be in contact with a larger toothed wheel carrying a cylinder, _E_, over which passes another smaller chain. This chain, passing along the upper surface of the cylinder, _A_, and round a second cylinder, _F_, at its further end, is acted on by a spring so as to be kept in a constant state of tension. In the middle of this chain a small tube is fixed for carrying a pencil, which, being gently pressed down by means of a small weight on the top of it, performs the duty of marking on paper placed round the cylinder the progress of the rise or fall of the tide as the cylinder revolves, and as it is drawn by the chain forward or backward by the rise or fall of the float. The paper is prepared with lines equidistant from each other, to correspond with the hours of the clock, crossed by others showing the number of feet of rise and fall. [Illustration] The cylinder while in action revolves from left to right to a spectator facing the clock, and the pencil is carried horizontally along the top of the cylinder; and the large wheel being made to revolve by the rise and fall of the float, turns the wheel with the small cylinder, _E_, attached to it. If the tide is _falling_, the small chain is wound round the cylinder, _E_, and the pencil is drawn towards the large wheel; but if the tide is _rising_, the small chain is wound on to the cylinder, _F_, by means of the spring contained in it, which constantly keeps it in a state of tension. Thus, by means of the rise and fall of the tide, a lateral progress is given to the pencil, while the cylinder is made to revolve on its axis by the clock, so that a line is traced on the paper showing the exact state of the tide continuously, without further attention than is necessary to change the paper once every day, and to keep the pencil carefully pointed; or a metallic pencil may be used, which will require little, if any, attention.