whole series. In the United States, the Virginia and the Washington, the deviation also is very great. Much remains yet to be done to perfect this important part of the subject.
'Alany of the boats on the Mississippi have their wheels abaft, that they may be protected from the logs of timber incessantly floating on that mighty river, thus practically exemplifying the original idea of Hulls. Many vessels also, intended only for short passages, and where a small draught of water is necessary, are built with two bodies, with the wheel placed between them. This plan, however, is not found advantageous for boats with any considerable draught of water, be cause, in addition to their weakness, there is an in crease of resistance resulting from the water passing with great velocity through a confined channel. Boats have been tried with two pair of paddle wheels, and the Dutch are now building a steam frigate with four engines, qach of 100 horse power, to act on two pair of paddle wheels.
When there are two paddle wheels on each side, their relative velocities, with respect to the water, should be equal, in order that they may exert an equal force on the vessel. If this were not the case, the af termost wheel would operate disadvantageously: for as the water on which the aftermost wheel acts, has had an increased velocity communicated to it by the action of the foremost wheel, the absolute velocity of the aftermost wheel must be proportionally greater than that of the foremost; a circumstance which would require a greater quantity of steam, and consequently a greater consumption of fuel. There would also be a waste of power, unless each pair of wheels had separate engines: and it is probable that the aftermost wheels would lose a portion of their effect, in consequence of the disturbed state of the water they acted on.
The following important table was communicated to M. Mnrestier by one of the principal engineers of New York, as the result of his experience with regard to the proportions between the dimensions of a vessel and its engine; and, in order to make this part of his useful and important work as complete as possible. he has added another table, the result of his own in quiries, containing the principal proportions of the engines and paddle wheels, Sec. of the steam boats, the
dimensions of which have been given in the preceding tables.
This table proves that the position of the paddle axis is very variable in different vessels. In the Chan cellor Livingston it is placed in the middle of its prin cipal axis, and very nearly so in the Fulton; but in the Philadelphia of Baltimore, the deviation from the centre is very considerable, and the greatest of the If the different boats were equally perfect in their respective elements, there would be no necessity for different multiplier for each boat; but, as the forms of their bodies, and the qualities of their engines dither considerably, the multipliers must necessarily vary.
I. Alarestier that the variation for the first nine boats recorded in the table is between twenty and twenty-live. The Savannah he did not include in his computation, as he had no precise information re specting her.
AI. Marestier has also given the following compa rative table of the results he has observed, and calcu lated for ten boats, of which he was able correctly to ascertain the velocities.
In the first column, the measure of the dustieity cf the steam, is represented by the height of the column of mercury it will support in a vacuum.
The column devoted to the proportion of the paddles, is the quotient of the rectangle of the breadth and draught of water of the boat, divided by the area of one of the paddles.
The number he terms the factor of the diame ter of the wheels, he obtained by considering, that If the vessels were similar, and the resistances to the paddles bore in all of them the same invariable rela tion to the resistance of the hull, the diameter of the paddle wheels would be equal to the velocity of the boat multiplied by a constant factor, and divided by the number of double oscillations of the piston. The mean of these Factors being between 29 and 50, it fol lows, that if the proportion the velocity of a steam boat bears to the number of strokes of the piston, be multiplied by 29 or SO, the result will give nearly the dimensions of paddle wheels similarly proportioned to those in the American boats.