( * 4 9 ) 
ioo, whereas an Elongation of 90 Deg.' gives the 
Diftance of the Apex from the Sun equal to that 
of the Earth at the time of Obfervation. 
This Seftion is clofed with an Account of the 
Changes, both real and apparent, to which the Zo- 
diacal Light , or Solar Atmofphere is liable Its 
Length has been for fome time upon the Increafe, af- 
terwards in a diminifhing Condition, and has been 
altered fo much in the Compafs of thirty-feven 
Months, as to have been 30 Deg. longer at one time 
than at another. The Changes in Luminoufnefs, 
Denfity and Tranfparency, has likewife been found 
to be very confiderable. Sometimes the Zodiacal 
Light has been fo rare and weak as to be but juft 
vifible, afterwards for a long time not vifible at all. 
Hereupon our ingenious Author thinks proper to 
obferve, that thefe Confiderations may ferve in fome ‘ 
Meafure to account for thelnconftancy of the Aurora 
Borealis , asalfofortheirNon-appearnce for fome Years? 
fincethey owe their Original to, and have fo clofe a Con- 
nexion with the Zodiacal Lights whofe Appearance is 
fo uncertain. Add to this \heZodiacal Light t as he af- 
terwards Ihews, muft not only be of a fufficient Length 
and Denfity, but the Earth muft be in or near the 
Nodes, form’d by the Interfe&ion of the Plane of 
the Sun’s Equator with the Plane of the Ecliptick. 
The fecond Seftion treats at large of the Altitude 
of our Atmofphere, and of that of the Region in it 
ufually poflefs’d by the Aurora Borealis. Under 
this Head he difcourfes of the feveral Methods the 
Mathematicians have ufed to find the greateft Heights 
of the Air, fuch as the Duration of the Twiiight, 
the Altitude of the Mercury in the Barometer, and 
Kk 2. rejt£b 
