S4 Mr Adamson 091 Rail-Roads. 



some of those parts which we could detach ; and thus, by the ef- 

 fect of those which remained, judge of the proportionate influ- 

 ence of each of them. Mr Wood has narrated an experiment 

 made for the purposes of ascertaining the total friction of an un- 

 loaded engine ; and from the additional retardation caused by it^ 

 when attached to waggons descending an inclined plane, he esti- 

 mates the friction of its joints, axles and pistons, to be no more 

 than 91S lb. Now, the resistance, by the friction at the axles 

 of the wheels, could not, according to the lowest estimate in the 

 table of experiments or friction, have been less than 100 lb ; 

 so that only 113 lb. remain as the retarding force of the pistons, 

 and other parts of the machinery. 



Another method of estimating this retardation, is afforded by 

 the experiments with wheels of different sizes. It was found, 

 that, by applying to the same engine wheels of different diame-* 

 ters, diff'erent results were produced by the same expenditure of 

 motive force in the same time. The retardation being equivalent 

 to a constant pressure acting through unequal spaces, must have^ 

 required, to overcome it, an expenditure of force in proportion 

 to these spaces, which are as the diameters of the wheels. The 

 resistance opposed by the rubbing parts would, therefore, when 

 3 feet wheels were exchanged for 4 feet wheels, be diminished . 

 in the proportion of 4 : 3 ; or the observed increase of effect frmn 

 the same pressive power, must have arisen from the annihilation 

 of one-fourth of the friction, by the addition of one-third to the 

 diameter of the wheels. The increase of eff^ect appears to have 

 been equivalent to a force of 146 lb. * ; and, therefore the total 

 friction of the engine with 3 feet wheels amounted to 584 

 lb. If from this we deduct the 100 lb., which will represent 

 the constant resistance at the axles of the wheels arising from 

 the weight of the engine, we shall have 484 lb. as the measure 

 of the resistance from friction, in all the other parts of the en- 

 gine. The measure of this retarding force in the former case, 

 when the engine was unloaded, was 113 lb. These two num- 

 bers tannot yet express the ratio according to which the friction 

 increases as the load is augmented, for the friction created by 



• This is greater than Mr Wood's estimate, and is found b^ taking the 

 2^ part of the additional load the engine carried with the' same fJLiel»_, ,. 



