( 339 ) 
And now I have this Co add about the Conftrufiion of this 
Inftruccent , that perhaps it would not be very difficu t to pro- 
pofe a niuch more accurate and elaborate Contrivance,if 'twere 
thought fit to propound any that would require a^nextraordi^ 
nary skill in the Artificer to make it, and fome confiderable 
skill or dexterity in thePerfon that is to ufe it i but the flight 
Conftrufiion, hitherto <defcribed, feemM to fuit better with 
niy Principal aim, which was, to propofe at prefent an Inftru- 
ment as fimple, cheap, and eafie to be employed and kept in 
order, as I could well examine Guhys mih ; little doubting 
but that the Principle, upon which this is framed, being well 
underftood and confidercd,will,if it be found ufeful, be further 
improved by new Applications and more Artificial Contri- 
vances, 
Explication 
