124 Examination of Mr. Quinby’s Crank Motion. 
mentioned to a number, considerably older than myself, that 
I visited yesterday one of the greatest curiosities in New- 
England, and when told what it was, they have replied with 
a smile, “I have always known the floating island in the 
meeting-house pond,” 
Arr. XVI.—Evxamination of Mr. Quinby’s Principle of 
Crank Motion. — 
THERE is contained in the 7th Vol. of the ‘* Journal of Sci- 
ence and Arts,” a paper by Mr. Quinby, of N. York, on the 
subject of crank motion. He has undertaken to show that 
there is no loss of power in communicating rotatory motion 
by means of the crank ; and in the endeavor to prove this 
assertion, he has so blended correct mechanical principles 
ith incorrect, that at first sight, his demonstration appears 
plausible: a careful examination cannot fail to convince any 
one, even slightly acquainted with mechanics, that however 
ingenious the solution may be, it is in point of fact incorrect. 
The proof is based upon the well known proposition, that 
when a weight in descending causes an equal weight to as- 
cend, through a space equal to that gone over by the de- 
scending weight, there is no loss of power. After announc- 
ing this fact, Mr. Quinby sets out to show that if the shackle 
_ bar acted always in a direction parallel to that of the piston 
rod, there would be no loss of power. Let us refer to his de- 
monstration : the circle v/Gw (fig. 1) is constructed so that 
its radius CG shail be a third proportional to the quadrant 
AD and radius CD, of the circle ADBE, representing 
ihe circle in which the lower end of the shackle bar moves = 
he then shows that the mean tendency to rotation in the wheel, 
caused by equal powers, acting at the different points of the 
wheel, in directions parallel to that of the piston rod, is 
equivalent to a constant force, (equal to each of these pow- 
evs acting on the crank,) acting at the point G of the circle 
welG, in the direction PG; then concludes,—since P in 
descending through the space Py raises W equal to P, 
throvigh a space Wx equal to Py, there is no loss of power. 
This is certainly a very ingenious argument, but it can 
not stand the test ofexamination. The weight P, at the dis- 
