( 17 ) 
of all the Modern Accuracy. For thefe Obfervations 
give the length of the Tropical Year fuch as to antici* 
pate the Julian Account only one day in 300 Years ; 
but we are now fccure that the faid Period of the Sun's 
Revolution does anticipate very nearly three days in 
400 Years ; fo that the t^ihles of Ptolemy founded on 
that Suppofition, do err about a whole day in the Sun's 
Place, for every Years. Which principal Error in 
fo Fundamental a Point, docs vitiate the whole Super- 
flru(aure of the Almagefty and ferves to convifl: its Au- 
thor of want of Diligence, or Fidelity, or both. 
But to return to our Method, the great Advantage 
we have hereby, is, that any very high Building: ferves 
for an Inftrument, or the top of any hig^i Tovyer or 
Steeple, or even any high Wall whatfocver, that may 
be fufBcient to intercept the Sun, and caft a true fliade: 
Nor is the Pofition of the Plane on which you take the 
lhade, or that of the Line therein, on which you mea=» 
fure the Receft of the Sun from the Tropick, very ma- 
terial; but in what way foever you difcovcr it, the faid 
Recefi will be always in the fame Proportion, by rea- 
(bn of the fmallnefs of the Angle, which is not fix Mi- 
nutes in the ffrft five days : Nor need you enquire the 
height or diftance of your Building, pravided it be ve- 
ry great, fo as to make the Spaces you meafure large 
and fair. But it is convenient that the Plane on whicii 
you take the fliade be not far from Perpendicular to the 
Sun, at lead not very Oblique, and that the Wall.which 
cafts the (hade, be ftraight and fmooth at top, and i\s 
Direftion nearly Eaft and Weft, for Reafons that will 
be well underftood by a Reader skilful in the Doftrine 
of the Sphere. And it will be requifite to take the Ex- 
tream greateft or leaft deviation of the lhadow of the 
Wall, becaufe the lhade continues for a good time a$ a 
Hand, without alteration,, which w^ill give the OMer** 
ver leifure to be alEired of wkat he does, and not be 
furprize^ 
