367 
Steam Engine, 
moving the machine. And farther, the greater the radius 
of the horse- walk, the nearer the chord of the circle in 
which the horse draws is to coincidence with the tangent, 
which is the most advantageous position of the line of 
traction. Hence it follows, that although a horse mag 
draw in a walk of 18 feet diameter, yet he will work with 
far greater ease in one whose diameter is 35 or 40 feet ; 
and it is very desirable that an experimental enquiry should 
be made to ascertain the proportion and absolute quantity 
of work in different circles. 
I am of opinion that it would not be difficult to make 
some useful experiments, while work was actually carry- 
ing on at any horse mill, or machine where horses are 
constrained to move in a circular walk. The simple 
drawing which accompanies this letter will assist in con» 
veying a clear idea of the method which I fancy might be 
advantageously adopted. Let AB Fig, 1. Plate IV. be 
the vertical shaft to which the horizontal horse-poles AC, 
AD, are attached. Let one horse work the machine by 
drawing at the ear E ; but, instead of the transverse bar 
to which the harness is fixed being simply hung upon 
the hook A, let a good spring steelyard be interposed 
between that cross-bar and the hook, the graduations of 
which shall, when the machinery is put into motion, in- 
dicate the resistance (in lbs.) overcome by the animal, in- 
cluding the weight of the mass moved, the friction, &c. 
Near the extremity of the opposite horse-pole AD, let 
there be fixed a strong and correct common steelyard, 
whose divisions shall shew the various weights from 40 
or 50 to 2001bs. and whose centre of motion shall be at 
the point f on the fixed stand. Let the cord c which is 
fastened to the shorter arm of this steelyard, pass (with as 
little friction as possible) over the pulley />, and thus, being 
turned into the horizontal direction, or rather, inclining a 
little upwards, let it be fixed to the cross bar of the har- 
