GEODÆSIA Improved. 277

It is affirmed that the Needle has a particular diurnal Varia-tion of a few Minutes, namely, from about eight in the Morn-ing, till about four in the Afternoon ; during which Time theNeedle is gradually and regularly affected by a westerly Motion,and then becomes stationary for some Hours: The Variationwestward gradually decreases, and the Needle re-affumes its for-mer State of Correctness. This Variation increases and decreases,as the Sun approaches to or declines from us, viz. it is supposedto have in

'January 7")February 9 1Ma r ch 11 IApril 12 yMay 13 I'June X 3 J

Minutes,

lJub

AugustSeptember^ OiioberI NovemberJ December

*3 r

12111087

'S Minutes.

Hence the Variation of the Needle (in the Surround of a Sur-vey) may more justly be imputed to this, than to the magneticVariation, for 1 can assure my Reader that the Variation of theCompass (as it is commonly called) has not the least Effect uponany Angle taken by a Needle in the Field; for Reason will con-vince us, that as the Point of Attraction is so very remote fromthe Instrument, the Error of Course is insignificant; and though500 Chains were measured easterly, or westerly, from any Me-ridian in the temperate Zone, it would not affect the Niedle onetenth of a Degree, or 6 Minutes, with respect to the Angle.

Thus have I given my Reader a brief easy Method of findingthe Variation, which will enable him not only to place his Mapin a true Noah and South Position (when the Needle is used,)but hkiwife v if he has any Notion of Dialling) direct him howto six his Meridian, or twelve o’CIock Line.

CHAP.