MINUTES OF PliOCEEDINGS OF 
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by surfaces to begin to slide; while for the purpose of determining data 
which should guide the construction of breaks or compressors the action of 
surfaces when held tightly in contact and during their motion must be the 
aim of the experimenter. 
Prom the construction of most compressors and breaks, particularly such 
as employ wood surfaces, the actual compression of the material is an essential 
element in their action, in addition to friction and adhesion, while the latter 
from the violence of the commencement of motion, is of the least possible 
value. 
In order to approximate to the conditions under which compressors act, 
long tongues or slips of material were pressed between stationary blocks 
having smaller surfaces, and were set in motion by a blow. The lowest 
weight which would sustain the motion when once set up gave the value of 
the resistance. 
The blow was used to destroy adhesion, and, as will be seen in the de¬ 
scription of the experiments, any increase in the intensity of the blow, beyond 
that absolutely required to establish motion, did not in the least affect the 
value of the resistance, merely accelerating the motion during the instant 
of contact. 
j Description of the apparatus . 
A lithographic press was selected as offering advantages in applying and 
removing the pressures. The bed and roller were removed, and it was fitted 
up as shewn in Plate I. 
A long lever c, resting upon a steel knife-edge fulcrum at d , acted with 
its shorter arm upon the under side of a table w, pressing it upward with a 
pressure proportional to the weight applied at the opposite arm. 
The weight of the power arm was composed of the weights, the scale pan, 
and the lever itself. 
On the table the specimens (as e,fe) w r ere placed; the upward pressure 
of the lever brought the upper surface in contact with the cross beam a , 
which was capable of being raised or lowered by means of the screw and 
lever b , r. 
Strong buttresses h } and cross pieces g, prevented the blocks e , e, intended 
to be stationary, from moving when the tongue f was pulled through by 
means of the chain i , the latter being fixed to the pulley h. 
In order to lessen the number of weights used in drawing the pieces 
through, a segment of a large wheel l was fixed upon the same axle as the 
pulley, a counterbalance weight m being used to prevent its weight in¬ 
fluencing the result. The scale pan was attached to the circumference of 
the segment, as shewn. 
To measure the comparative values of the blows used to set up motion, a 
ball pendulum o was used, together with a card board sector q. 
Dimensions^ weights 3 Sfc . connected with the apparatus . 
Weight of lever c, 44-375 lbs. 
Length of arms = short, 3 in., long 60 in. 
Distance of centre of gravity from fulcrum 29T2 in. 
