34 APPENDIX. 



It is however expedient to keep a statement of its operations, the benefit of which will be, 

 that the officer will gain a practical knowledge of the effect produced by the partial opening 

 or closing of the valves, when, in a high sea, the ship maybe overpressed with steam power, 

 a consideration of the utmost importance; which, if leading to judicious management, may 

 prevent incalculable mischief in the rapid deterioration of the hull and machinery, besides 

 the possibility of immediate danger. The steam journal will form a permanent record of the 

 way this duty has been attended to. 



The course of the ship, her progress, and the direction of the wind, should also be entered 

 in the journal, as the angle between the first and third will indicate the proportionate degree of 

 resistance which the wind affords ; while the numerical symbol in the column of " Force " 

 marks its power, and the variation in the rate of progress forms the estimate of the effect. 



If the table before described is used, a column should be ruled next the " Knots" for the 

 purpose of marking the progress of the " Centre of Effort," the variations between which and 

 the vessel's speed will form the basis of considerations most useful in estimating the effective 

 duty of the engine. As the measure of USEFUL effect or duty of the marine engine is, in fact, 

 the rate at which the vessel, under given circumstances, is moved through the water, the 

 area of the paddle should be such as to displace the fluid (when acted upon by the maxi- 

 mum speed of the engine in still water at the mean trim of the ship) at that rate most 

 favourable to propulsion by its reaction. A few experiments in each vessel would decide 

 the proportion which the speed of the ship should bear to that of the " Centre of Effort ;" 

 these experiments of course being made under circumstances of known and unvaried re- 

 sistance, in considering which particular reference should be had to the ship's immersion 

 in the water. In closing this subject, it is necessary distinctly to state, that the position 

 previously given for the " Centre of Effort," and also stated at the foot of the table, is 

 only to be considered as an approximation, which may indeed be too far from the truth to 

 admit of that appellation ; still it is presumed that for practical purposes it is sufficient, 

 though by no means meant as an absolute datum. 



found materially to alter the assumption offered above ; neither can it affect the principle, that the power is not 

 lessened in the same proportion as the aperture of the throttle is diminished; although it still affords the means, if nearly 

 closed, of using steam of low pressure in the cylinder when it is desirable to decrease the power. 



