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II. T/je Croonian Lefture on Mufcular Motion . 
By George Fordyce, M. D . F. R . S. 
Read November 22, 1787. 
T HE fubjedl of mufcular motion has been fo often con- 
fidered, and in fo many lights, that it is hardly pofilble 
to avoid many obfervations which are trite, and even puerile ; 
which, when they occur, I beg the learned Society to forgive. 
They will alfo find fo little new, or even nothing, that I 
fhould be inexcufable in taking up their time, if the fubjedt was 
not annually to be difcufifed before them. In confidering muf- 
cular motion, I muff begin with fome obfervations on motion 
in general, and with that well known, and felf-evident axiom, 
that one particle of matter, confidered by itfelf, will remain 
at reft if it be at reft, and will continue in motion if it be in 
motion, and in the fame direction. This has been called the 
vis injita , or vis inertia, of matter. It may be faid in other words, 
that a fingle particle of matter being at reft, would therefore 
always continue at reft, if it were not for fome external im- 
pulfe made on it. This impulfe may be from fome other par- 
ticle of matter in motion impinging upon it, and communi- 
cating part of its motion to it, while it communicates an equal 
quantity of its reft to the matter fo impinging upon it, fo that 
the quantity of motion and reft (hall be the fame after the 
impinging, in both bodies, as they were before : or, in other 
words, a fimple particle of matter in motion would always 
continue 
