RULES FOR CALCULATING THE STEAM ENGINE. 231 



The velocity would, therefore, be about 57i miles per hour; but this is sure to be in 

 excess, since at such high velocities the resistance offered by the atmosphere must be very 

 considerable. 



For any particular engine the constant number, which forms the dividend, when once 

 prepared, will be ready for general application. 



In the calculation of the average speed, the distances traversed, or the times of transit, due 

 allowances should be made for the intervals spent in the generation of motion. Nearly 

 one-half of the time elapsed, or one-half of the distances passed over, in the gradual pro- 

 duction and annihilation of the velocities, may be considered as lost. 



FORMULAE for calculating the effects of the SCREW PROPELLER, when applied to 



steam vessels. 



r = the radius of the screw in feet. 



h = the pitch, also in feet. 



TT = 3-14159 log. TT = 0-49715. 



log. c =7-75506. 



2 ff 550 



log. 2 IT - 0-79818. 



2 7T 



v = the velocity of the vessel in feet per second, or the velocity in miles per hour mul- 

 tiplied into if. 



u = the velocity of a point in the screw at the distance k from the axis. 

 S = the effective resisting surface of the vessel. 



Number of revolutions per second = . 



h 



Horse power of engine = c A (u v) 2 . u 

 Horse power effective = c A (u v) 2 . v 



In the absence of better information, the value of S may be roughly estimated at about 

 one-fifth or one-sixth of (tonnage)T, according to the construction of the vessel. 



The larger the diameter and the less the pitch of the screw, the greater will be the pro- 

 portion of effective power on the vessel. 



