FORM OF STEAM JOURNAL. 33 



or any defined spot on the radii of the paddle wheel, with the object of pointing out 

 the difference between the velocity of the paddle, which by its reaction on the water 

 propels the ship ; and the speed of the vessel thereby produced. As it would be unreasonable 

 to use for this purpose the extreme periphery of the wheel, which is so much immersed, 

 so also it is not deemed proper to take that point on the radii which corresponds with the 

 water line ; but it is proposed that the velocity of the paddle shall be understood to mean 

 the velocity due to the rotation of the wheel, of that point on the paddle about which all the 

 propelling force is concentrated in the various positions it assumes, from its entering to 

 its leaving the water, which may be called " the Mean Centre of Effort." The variable 

 immersion of the paddle in each rotation of the wheel, together with other conditions that 

 should be taken into consideration, render it extremely difficult to ascertain the exact 

 distance, from the outer edge of the paddle, where the mean horizontal effort is collected, 

 and the question involves the most abstruse mathematical investigation ; it is however near 

 the centre of percussion ; and, as an approximation, we may, for practical purposes, assume it 

 to be in Morgan's wheel at the centre of the paddle, and in the radial or old wheel at one- 

 third of the board from its outer edge : thus in the latter case, supposing a wheel of 16 feet 

 diameter, having boards 18 inches wide, the centre of effort may be assumed to be 7 feet 

 6 inches from the axis ; and on reference to the table, we shall find that if a wheel of the 

 above dimensions be making 21 revolutions per minute, the centre of effort is, in reference 

 to the revolution of the wheel, travelling at the rate of 9f miles an hour ; the difference 

 between which and the speed of the vessel would be uniform, if the resistance was unvaried. 

 It is proposed to note hourly, in a column next the " Knots," the rotary progress of " the 

 Centre of Effort" in miles, derived from inspection of the table. 



It is necessary to make a few observations with reference to the throttle-valve, which, as 

 its use is to controul the admission of steam to the cylinders, is supposed by most steam 

 seamen to do this so effectually, that a record of its partial closing or opening forms a just 

 estimate, in all cases, of the power acting on the engine ; but that this is not the case, may 

 be exemplified by the two following hypotheses. First, supposing the ship in good trim, the 

 weather calm, and the sea smooth, the throttle-valve being quite open, the engines will 

 attain the maximum velocity, which may be assumed at 22 revolutions per minute, and the 

 speed of the ship at 10 knots an hour. Secondly, the vessel's trim may be the same, the valve 

 still open, but the violence of the wind and the sea may have so increased, that the rate of 

 the engine is reduced to 11 revolutions a minute, and the speed of the ship to 4 or 5 knots 

 an hour : it is now evident that the consumption of steam in equal times is about half what 

 it was in the former case ; it would be exactly half, except for the additional waste in the 

 cylinder, and other slight causes. It is also evident, that if the valve be half closed, it 

 would not diminish the quantity of steam used in the cylinder, or the power of the engine ; 

 thus, supposing a repetition of the first case when the ship is at full speed, it will be 

 found that the valve must be nearly closed to effect any material reduction of the velocity. 1 



> When an engine meets with any sudden increase of resistance, the steam will at first be spent at a higher 

 pressure and greater density, even if the safety valve acts freely ; but as on the occurrence of such cases at sea, the 

 draught is always diminished by the damper, and the fires slackened, this effect must soon cease, and it will not be 



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