( s r-)

greifest East or West Azimuts ; it may be then convenient to fix (ve-ry lärm and • stcadÜy on lömc Tower, Steeple, or orber high Editicc,

( in a convenient Situation) a good Telescopick Object Glast, in suchPosition as may be proper for viewing that Star. And at a due Di-stance from it near the Ground, build on purpole (if already there benot any) sonte little Stone-Wall, or like Place, on whicli to six the■Ryc Glast, so as to anstver tltat Object; Glast : And having so adjust-ed it, as through both to sce that Star in its desired Station, ( whichmay best be done vvhile the Star is to be soen by Night in such Situ-ation, near the time of one of the Solstices) let it be there fixed sofirmly, as not to be disturbed, (and the place so lecured, as that nonecome to disorder it) and care be taken so to defend both the Glases, asnot to be endangered by Wind and Weather. In which Contrivance, I- am besolden to Mr. John Caswcll, M. A. of Iiart-Hall in Oxford , for hisAdvicc and Ailistancc, with whom I have many years since communicatedthe whole matter.

Tliis Glast being once fixed, fand a Micrometer fitted to it, so asto have its Threads Perpendicular to the Horizon, to avoid any incon-venicncc which might arife from Diversity of RefracHon if any be) theStar may then be viewed from time to time (for the following year or lon-ger) to see if any Changc of Azimuts can be obsorved.

This I thought fit to Recommend to your Conilderation, who do sowell understand Telcseopes, and the Managery of them : But when Iiiiggest, (as a convenient Star for this purpole ) the Sboulder of theLejscr Bear, (as being the nearest to the Pole ot the Zodiack of anyStar that is of the First or Second Magnitude) I do not confine you tothat Star ; but (without Retracting that) Suggest another • namely, theMiddlc Star, in the Lall of the Great Bear, which (though somewhat fur-ther from the Pole of. the Zodiack) is a brighter Star than the other, and. may be nearer to us.

But I do it principaliy upon this Consideration ; namely, That thereis adhering to it a very (mall Star, (which the Arabs call Alcor, ofwhich they have a Proverbial läying, when they would Dcsoribe a (harpSighted-Man, That he can diseern the RJder on the Middlc Harfe of theTVtyn ; And öf one who pretends to sce fmall things but overlöoks muchgreater, Vidit Alcor at non Lunam Plenam : ) Which Hevelius in his Ob-iervarions, finds to be Distant from it about nine Minutes, and five orten Scconds : So that besides the Advantage of difeovering the Parallaxof the greater Star, if discemable ■ the Difference of Parallax of that andof the leiser Star (being both within the reach of a Micrometer) maydo our Work as well. For if that of the greater Star be Discemable,hüt that ot the leiser be either not Discemable or lest Discemable, theirdifferent. Distances from each other at different times of the Year may'perhaps (without tarther Apparatus ) be Disoerned by a good Telescope ofa competent lcngth, furnishcd with a Micrometer, is carefully - preferved 1