( i8 ) ■ 
furprized by the quick tranfient motion of the fliade of 
a fingle Point at fuch a diftance. The Principal Obje- 
St\on is, that the Venumlra or Partile fliade of the Sun 
is in its extreams very difficult to diftinguifli from the 
true fliade, which will render this Obfervation hard to 
determine nicely. But if the Sun be tranfmitted through 
a Telefcope^ after the manner ufed to take his Species in 
a Solar Eclipfe, and the upper half of the Objeft-glafs 
be cut off by a Paper pafted thereon, and the exad up- 
per Limb of the Sun be feen juft Emerging out of, or 
rather continging the Specks of the Wall, ( the Pofi- 
tion of the Te/efcope being regulated by a fine Hair ex- 
tended in the Focus of the Eye-glaft,) I am afliired that 
the limit of the fliade may be obtained to the utmoft 
exaftnefi : And of this I defign to give a Specimen by 
an Obftrvation to be made in June next, by the help of 
the High Wall of St. Paul's Church, London^ of which 
fome following Tranfaftion may give an Account. In 
the mean time what I have premifed may fuffice to fet 
others at work, where fuch or higher Buildings are to 
be met with. I ftiall only Advertife, that the Winter- 
Tropick by this Method may be more certainly obtained 
than the Summer Sy by reafon that the fame Gnomon does 
afford a much larger Radius for this manner of Obfer- 
vation, 
III. jh 
