RULES FOR CALCULATING THE STEAM ENGINE. 229 



*#* The two preceding rules are founded on the Chev. de Tambour's estimation of the 

 resistances experienced by locomotive engines, stated at the foot of page 185 ; which are, 

 doubtless, good approximations to the truth with the usual proportions and velocities of 

 locomotives ; but it should be observed, that in the determination of these resistances, the 

 experimental data are all necessarily affected by the resistance of the atmosphere, which varies 

 nearly as the square of the velocity. It is evident that the results must involve an average 

 value of the atmospheric resistance for the experiments adopted, and that no material cor- 

 rection will therefore be required, on account of the atmosphere, with ordinary velocities, 

 though some additional resistance should be added for very high velocities. It may also be 

 a question whether the resistance of the carriages, &c., would at all be affected by altering 

 the diameters of the running wheels. 



29. To determine the power of a locomotive engine. 



RULE. Find the effective pressure on the piston by the last rule, and thence the power ^ , 

 by the rule of No. 24. 



The following method, however, will be found to be more convenient and useful in practice. 



Calculate the resistance by the last rule ; multiply by 8 times the velocity in miles per 

 hour ; point off 3 figures from the right hand, and divide by 3. The result will express the 

 number of horses' power. 



Example. Take the last case, and suppose the velocity to be 12 miles an hour. 

 The resistance has already been calculated, 1935 Tbs. 



Resistance 1935 tfes. 



8 times speed 96 miles. 



11610 



17415 



3) 185-760 



61-92 horse power. 



30. The power of a locomotive engine being known, it is required to determine the speed 

 under any given circumstances of load, inclination of railway, &c. 



RULE. Find the whole resistance in ffis. overcome, by the rule of No. 28 ; multiply the 

 number of horses' power by 375, and the product will be a constant number to be divided by 

 the resistance to obtain the velocity in miles per hour. 



Example. Taking the same case as before, we have, 



Horse power 61-92 

 375 



30960 

 43344 

 18576 



Constant number 23220-00 



