that whatever is faid of one may be faid of t'other. 
However I think to give fometime a more formal De- 
monfl ration of it. 
See the Theorem Fig, 5-. 
IV. A Demonjiration of an Error committed by 
common Surveyors in comparing of Surrueys 
taken at long Inter*z/ah of Time arijing from 
the Variation of the Magnetir\ Needle by 
William Moiyneux Efq-^ F. R. S. 
TH E Variation of the Magnetick Needle is fo 
commonly known^ that I need not infift much 
on the Explication thereof, 'tis certain that the true So* 
tar Meridian, and the Meridian flievvn by a Needle 5 
agree but in very few places of the World ; and this 
too, but for a little time (if a Moment) together. The 
Difference between the true Meridian and Magnetick 
Meridian perpetually varying and changing in all Places 
and at all Timps ; fomeiimes to the Eaftward and fome- 
times to the Weftward. 
On which account 'tis impoffible to compare two Sla- 
veys of the fame place, taken at diftant times, by Mag- 
netick Inftruments (fuch as the CircHmferentor^y which 
the Down Survey^ or Sir William Pettf s Survey of Ire-^ 
land was taken) without due allowance be made for 
this Variation. To which purpofe we ought to know 
the Difference between the Magnetick Meridian andtrue 
Meridian at that time of the Down Survey, and the faid 
Difference at the time, when we make a New Survey to 
compare with the Down Survey, 
