106 
Rev. Mr Adamson on the extent of our 
in use. 1 ’ Now, this forms the strongest objection to any reliance 
on his experiments ; for no model can have that relative smooth- 
ness, and accuracy of workmanship, which is found in larger ma- 
chines. In constructing coal-waggons, which carry about three 
tons, the minutest attention is generally employed to secure ac- 
curacy of form, and smoothness of surface, in the moving parts. 
I had an opportunity of observing the importance of attending 
to those circumstances, while assisting Mr Wood to make some 
experiments at Killingworth, in August last. In applying 
some new bearings to new axles, though both had been turned 
and polished in lathes with the utmost care, yet the friction at 
the axle did not become constant, and reach its minimum, until 
the carriages had been dragged about heavily loaded, during a 
whole day. It requires attention to those circumstances, to ac- 
count for the difference in the ratios of the friction to the weight, 
as determined by Mr Tredgold and Mr Wood, the one estima- 
ting the friction at double the amount which the other assigns 
to it. 
In Mr Wood’s table, representing the relative and actual re- 
sistances in different experiments, there is somewhat of embarras- 
sing obscurity, arising principally from his not having narrated 
before, the whole circumstances of one of the series of experi- 
ments contained in it *. It is satisfactory to find, that the re- 
sult of his two totally dissimilar methods of experimenting agree 
so closely. His deduction from them may apparently be de- 
pended upon in practice : That with wheels, of which the ratio 
of the diameter, to that of the axle, is 12:1, the total resistance 
will be part of the weight of the whole carriage and load. 
I have had an opportunity of witnessing experiments, in which, 
by taking every precaution to obviate the causes of retardation, 
it was reduced very considerably beneath the lowest amount in 
Mr Wood’s table. We do not yet know exactly what proportion 
the resistance arising from the contact of the wheel with the rail 
bears to the whole ; yet, as it is evidently very small, and proba- 
bly diminishes as the diameter of the wheel increases, we may 
decide, that the fraction of the weight expresses with suffi- 
cient accuracy the resistance at the axle, and that this quantity, 
Essay on Rail-roads,: p. 195. 
