♦ 
[ *39 ] 
This may be done two ways; either by caufing 
the air to move againft the machine, or the machine 
• to move againft the air. To caufe the air to move 
againft the machine, in a fufticient volumn, with 
fteadinefs and the requifite velocity, is not ealily put 
in practice : To carry the machine forward in a 
right line againft the air, would require a larger room 
than I could conveniently meet with. What I found 
moft practicable, therefore, was, to carry the axis, 
whereon the fails were to be fixed, progrefiively round 
in the circumference of a large circle. Upon this 
idea * a machine was conftrudted, as follows. 
Plate VI. Fig. i. 
ABC is a pyramidical frame for fupporting the 
moving parts. 
D E is an upright axis, whereon is framed 
F G, an arm for carrying the fails at a proper dis- 
tance from the center of the upright axis. 
* Some years ago Mr. Roufe, an ingenious gentleman of Har- 
borough in Leicefterfhire, fet about trying experiments on the ve- 
locity of the wind, and force thereof upon plain furfaces and 
windmill-fails : and much about the fame time Mr. Ellicott con- 
trived a machine for the ufe of the late celebrated Mr. B. Robins, 
for trying the refiftance of plain furfaces moving thro’ the air. 
The machines of both thefe gentlemen were much alike, tho’ at 
that time totally unacquainted with each other’s inquiries. But it 
often happens, that when two perfons think juftly upon the fame 
fubje£f, their experiments are alike. This machine was alfo built 
upon the fame idea as the foregoing; but differed in having the hand 
for the firft mover, with a pendulum for its regulator, inftead of a 
weight, as in the former ; which was certainly beft for the pur- 
pofes of meafuring the impulfe of the wind, or refiftance of plains : 
but the latter is more applicable to experiments on windmill-fails ; 
becaufe every change of pofition of the fame fails will occafion 
their meeting the air with a different velocity, tho’ urged by tha 
fame weight. 
T 2 His 
T 2 
