THE ROYAL ARTILLERY INSTITUTION. 
127 
Making allowance for the effect produced by repeated use, it appears 
from this series that the resistance varies directly as the pressures. In some 
cases it is a little more, and in others a little less, than this proportion; just 
as they increase or diminish their resistance from use. 
Series c. 
TO ASCERTAIN THE EFFECT OF VARYING- THE NUMBER OF SURFACES WITH A 
CONSTANT PRESSURE. 
The materials used were the same as those in Series b. The area of the 
bearing surface was also, in all cases, 32 square inches, and the pressure 
was maintained at | ton. 
The results are given in Tables XVIII. to XXVI; the latter being the 
general summary. 
From this series the important fact is ascertained that with a constant 
pressure the resistance of friction varies directly as the number of sliding 
surfaces upon which the pressure acts; thus with one piece moving there 
are two sliding surfaces, and calling the resistance a , this resistance will be 
2 a for two moving pieces, 3 a for three, and so on, although the pressure is 
unvarying. 
It is to this principle that the various forms of the American compressor 
owe their value, and by its application the pressure and consequent strain 
may be kept low, while the resistance may be increased to any extent 
whatever, by simply increasing the number of sliding surfaces. 
Series d. 
TO ASCERTAIN, WITH A VARYING PRESSURE AND USING NO BLOW, THE VALUE OF THE FRICTION 
OF “REST,” IN ORDER TO COMPARE WITH THAT OF “MOTION,” IN SERIES 5. 
The materials used were iron, surfaced and ground, and oak. 
The bearing surface was 32 square inches, as in Series b and c } and the 
pressure varied from 0’5 to 2 f 5 tons, as in Series b. 
The results are tabulated in Tables XXVII. to XXXIII. inclusive; Table 
XXXII. being a general summary, and Table XXXIII. a comparison with 
Series b (from Table XVII). 
It appears from these experiments that when motion was induced without 
a jar, the resistance was greatly increased, owing to cohesion setting in, but 
when once the weight in the scale pan was sufficient to destroy the cohesion 
the piece ran through in a much less time (J to Jth) ; wrought-iron 
through wrought-iron (surfaced) was, however, an exception, there being 
in this instance no practical difference between the friction of rest and 
motion; while ground iron through ground iron gave the greatest 
difference* 
