Royal Societv. ij^ 
To fundamental a point, does vitiate the whole fuperflru^lure of 
the almageft, ?.nd leems to convi£l its author either 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 lerves for an inftrumcnt, 
or the top of any high tower, or lleeple, or even any high wall 
whatever, that may be fufficient to intercept the fun, and cafl: a 
true fhadow ; nor is the pofition of the plane, on which you take 
the fliadow, or that of the line therein, on which you meafure 
the recels of the lun from the Tropic^ very material 5 but in what 
way fcever you difcover it, the faid recels will be always in the 
fame proportion, by reafon of the fmallnels of the angle, which is 
not fix minutes in the firft five days 3 nor need you enquire the 
height or diftance of your building, provided it be very great, {b 
as to make the fpaces, you meafure, fair and large. But it is con- 
venient, that the plane, on which you take the fhadow, be not 
far from perpendicular to the fun, at leaif, be not very oblique; 
and that the wall, which cafis the iliadow, be ftreight and fmooth 
at top, and its direction nearly eaft and weft; and it will be 
requifite to take the extreme greateft or leaffc deviation of the 
fhadow of the wall, becaufe the fhadow continues for a good time 
at a ftand, without alteration, which will afford the oblervcr 
leifure to be affured of what he does, and not be furpriled by the 
quick tranfient motion of the fhadow of a fingle point at iiich a 
diftance. The principal objeflion is, that the penumbra, or 
partile /hadow of the fun, is in its extremes very difficult to dil- 
tinguifh from the true fhadow, which will render this obfervation 
hard to determine nicely ; but if the fan be tranfmitred thro* a 
telefcope, in the ufual manner of taking his fpecies in a fblar 
eclipfe, and the upper half of the objef^-glals be cut off by a 
j)aper palted thereon, and the exa6t upper limb of the fun be 
leen juft emerging out of, or rather touching the fpecies of the 
wall ( the pofition of the teleicope being regulated by a fine hair 
extended in the Focus of the eye-glais) the limits of the fliadow 
may be obtained to the utmoft exadnels ; and by this ir.ethod the 
winter folftice may be more certainly had than the fummer, 
becaufe the lame gnomon at&rds a much larger radius for this 
manner of obfervation. 
7/:e 
