( 44 ) 
To give a clearer and fuller Apprehenfion of this 
Matter, our Author thinks it neceifary to alter the 
common Definition of Elafticity, and ufe the following 
new Terms : The natural Equilibrium of an Elaftic 
Body ; its violent Equilibrium ; and the Line of a tre- 
mulous Vibration. 
By the natural Equilibrium of an Elaftic Body, 
Mr. Gerjen means the external Figure and Extenfion, 
which an Elaftic Body naturally has, when free from all 
external PrelTure. By a violent Equilibrium, he un- 
derftands that State or Degree of Expanfion which an 
Elaftic Body is kept in by fome external compreffing 
Force ; and he calls that the Line of a tremulous 
Vibration, which a Point taken at Pleafure in an Ela- 
ftic Body defcribes during the Vibration. 
I pafs over, for Brevity fake, the Corollaries drawn 
from his Definition of Elafticity concerning the Ac- 
celeration and Retardation of a Vibration, as alfo the 
Defcription and Ufe of an Inftrument contrived 
to prove the Truth of what he had advanced; nor 
(hall I take any particular Notice of Prop. Ill, 
IV, V, and VI, wherein he treats of the Velocity 
of the Elaftic Bodies in their tremulous Vibrati- 
on, of the Motion of ftretched Strings, and their 
ifochronal Vibrations. 
In Prop. X he demonftrates, that fuppofing three 
Elaftic Bodies to be detained in a violent Equilibrium 
in the fame Line of tremulous Vibration, if the mid- 
dle be farther compreflfed on all Sides, fo as 
ftill to remain contiguous to the neighbouring Bo- 
dies, it may bereftored, by tremulous Vibrations, to 
a greater Extenfion than it had before. He then 
(hews 
