draught and trim, consists in attaching to it a skillfully devised
dynamometric apparatus secured to a lightly constructed carriage. This
carriage traverses a railway which extends the whole length of the
tank about 15 in. or 18 in. above the water. The floating model is
carefully guided in its passage through the water by a delicate
device, keeping it from deviating either to the right or left, but at
the same time allowing a free vertical and horizontal motion. The
carriage with the model attached is propelled by means of an endless
steel wire rope, passing at each end of the tank around a drum, driven
by a small stationary engine, fitted with a very sensitive governor,
capable of being so adjusted that any required speed may be given to
the carriage and model. The resistance which the model encounters in
its passage through the water is communicated to a spiral spring, and
the extension this spring undergoes is a measure of the model's
resistance. The amount of the extension is recorded on a revolving
cylinder to a much enlarged scale through the medium of levers or bell
cranks supported by steel knife edges resting on rocking pieces. On
the same cylinder are registered "time" and "distance" diagrams, by
means of which a correct measure of the speed is obtained. The time
diagram is recorded by means of a clock attached to an electric
circuit, making contact every half second, and actuating a pen which
forms an indent in what would otherwise be a straight line on the
paper. The distance pen, by a similar arrangement, traces another line
on the cylinder in which are indents corresponding to fixed distances
of travel along the tank, the indents being caused by small
projections which strike a trigger at the bottom of the carriage as it
passes, and make electric contact. From these time and distance
diagrams accurate account can be taken of the speed at which the model
and its supporting carriage have been driven. Thus on the same
cylinder is recorded graphically the speed and resistance of the