Mifcellanea Cur i of a. 283 



A c DemonJlration of an Error com- 

 mitted by common Surveyors in 

 comparing of Surveys taken at 

 long Intervals of Time arifing 

 from the Variation of the Mag- 

 netic^ Needle^ by William Mo- 

 lyneux. Efa\ F. R. S. 



THE Variation of the Magnetick Needle 

 is fb commonly known, that I need not 

 infill much on the Explication thereof; 'tis 

 certain that the true Solar Meridian, and the 

 Meridian fhewn by an Needle, agree but in 

 a 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 vary- 

 ing and changing in all Places and at all 

 Times ; fometimes to the Eaftward, and 

 fometimes to the Wefcward. 



On which account 'tis impofiible to com- 

 pare two Surveys of the fame place, taken at 

 diftant times, by Magnetick Inftraments, 

 (fuch as the Circumferentor , by which the 

 Down Survey , or Sir William Vettfs Survey of 

 Ireland was taken) without due allowance be 

 made for this Variation. To which purpofe 

 we ought to know the Difference between 

 the Magnetick Meridian and true Meridian 



'at 



