Memorandum on the Newcastle Engine Triab. 675 
at all events, not serious enough to affect the comparative results, 
whicli it was the main object of the trials to obtain. 
The point was not lost sight of when it became necessary to 
decide upon the mode of conducting the Newcastle trials. It 
was considered important that those trials should be carried out 
as nearly as possible in the same manner as those at Cardiff, and 
with the same instruments, in order that a comparison might 
be made which would indicate the advantages, or otherwise, of 
the compound system. 
It might be argued that if the sources of error were known, 
arrangements might have been made for measuring the pull on 
the ends of the adjusting levers, and obtaining in that way the 
means of making corrections if any were required. But any 
such attempt would have involved an important change in 
the brakes ; that is to say, the points of suspension of the 
levers would have had to be made moveable instead of fixed, 
aud the brakes would consequently not have been the same as 
those used at Cardiff. The importance of precise similarity is, 
in the eyes of the competitors, very gTeat, a fact which is illus- 
trated by the circumstance that Messrs. McLaren were not con- 
tent to be tried on a brake exactly similar to that assigned to 
Messrs. Davey, Paxman, & Co., but they wdshed to be run 
upon the very same instrument. Had time permitted, I should 
certainly have preferred to run all the engines on the same brake. 
The pull upon the upper ends of the adjusting levers depends 
upon two things : first upon the load suspended to the brake 
strap ; and secondly upon the mode of lubrication of the strap 
and wheel, because upon this depends the tightness of the 
strap. 
With a given weight of strap, and up to a certain tension, the 
amount of which depends upon the mode of lubrication, the strap 
does not come into actual contact with the whole circumference 
of the brake wheel. There is no appreciable variation of stress 
at the points of attachment of the brake strap to the adjusting 
levers, in consequence of variations in the load ; the brake strap 
does not press against the lower half of the wheel at all, and 
hence the levers are inoperative, that is to say, the swing of the 
lower ends will not alter the tightness of the strap, and there 
is consequently no pressure at their fixed ends caused by the 
load. When the lubrication is very good and the coefficient of 
friction is consequently small, the brake strap has to be tighter 
than when the lubrication is inferior and the coefficient of friction 
large, and the pressure on the fixed ends of the adjusting levers 
increases in proportion. 
As soon as the strap begins to bear all round the wheel, the 
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