236 
MicrocGRapuHaia, | 
has fometimes fo {lowly increafed, that I have obferved the Bubble to be 
{ome hours in pafling between the top and bottom. 
7. Whether the appearance of the Pzke of Tezerif, and feveral other 
- high Mountains, at fo much greatera diftance then feems to agree with 
ae ame 
their refpective heights, be not to be attributed to the Curvature of the 
vifual Ray, that is made-by its pafling obliquely through fo differingly 
Denfe a\‘Medium from the top to the eye very far diftant in the Horj- 
zon: For fince we have a I hope, made it very probable, that 
there is fuch an inflection of the Rays by the differing denfity of the 
parts of the Air; and fince Ihave found, by feveral Experiments made 
on places comparatively not very high, and have yet found the preffure 
fuftain’d by thofe parts of the Air at the top and bottom, and alfo their 
differing Expanfions very confiderable : Infomuch that I have found the 
preffure of the Atmo/phere lighter at the top of St. Paul's Steeple in Lon- 
dow { which is about two hundred foot high ) then at the bottom by a 
fixticth or fiftieth part, and the expanfion at the top greater then that at 
the bottom by neer about fo much alfo; for the Aéercurial Cylinder atthe 
bottom was about 39. inches, and at the top halfaninch lower 3 the Air _ 
alfo included in the Weather-glafs,that at the bottom fill d only 155. fpa- 
ces, at the top fill’d 158. though the heat at the top and bottom was 
found exactly the fame. witha fcal’d Thermometer: I think it very rational 
to fuppofe, that the greateft Curvature of the Rays is made neareft the 
Earth,and that the inflection of the Rays, above 3. or 4. miles upwards, is 
very inconfiderable, and therefore that by this means fuch calculations of 
the height of Mountains, as are made fromthe diftance they are vifiblein 
the Horizon,from the fuppofal that that Ray isa ftraight Line (that fom 
the top of the Mountain is, as'twere, a Tangent to the Horizon whence it 
is {een ) which really is a Curve, is very erroneous. Whence, I fuppofe,pro- 
ceeds the reafon of the exceedingly differing Opinions and. Afflertions of 
feveral Authors, about the height of feveral very high Hills. ass 
' 8. Whether this Inflection of the Air will not very much alter the fup- 
poled diftances of the Planets, which feemto have a very great de 
. dence upon the Hypothetical refraction or inflection of the Air, and that 
refraction upon the hypothetical height and denfity of the Air: For 
fince (as I hope ) Ihave here fhewn the Air to be quite otherwife then 
has been hitherto fuppos'd, by manifefting itto be, both of a vaft > at 
leat an uncertain, height, and of an unconftant and irregular denfity ; 
It mutt neceflarily follow, that its inflection muft be saat accordingly’: 
And therefore we may hence learn, upon what fure grounds all the A- 
{tronomers hitherto have built, who have calculated the diftance of the 
Planets from their Horizontal Parallax 5, for fince the Refraction and Pa- 
rallax are {o nearly ally’d, that the one cannot be known without the 
other, efpecially by any wayes that have been yet attempted, how uncer- 
tain muft the Paral/ax be,when the Refraction is unknown? And how eafie 
is it for Aftronomers to affign what diftance they pleafe to the Planets,and 
defend them,when they have fuch a curious ubterfuge as that of Refratti- 
‘ on,wherein a very little variation will allow them liberty enough to place 
the Celeftial es at what diftance they pleafe, 
