I 3^7 ] 



in the tube, &c. may by the firft explofion have been fo con- 

 fumed. 



But whether this or the contrary be the fadl, the experi- 

 ments have fatisfied me, that the Aurora EoreaUs is an elecfl- 

 rical phjenomenon ; that, (at leaft when it is faint and of a 

 pale colour) it cannot appear in air lefs rarefied than near 

 4000 times * ; and confequently that its nearefl diflance from 

 the earth is about forty- five miles (according to Docflor Halley's 

 table of the air's rarefacflion at different altitudes'; ; that in air 

 rarefied more than 26000 times, it would not be vifible, and 

 therefore its greatert diftance is about fifty miles, (by the fime 

 tablej : I am notwithflanding fenfible it may be lefs or greater : 

 it may be le/s, for though my pear-gage fhewed that degree 

 of rarefadlion, I pretend not to fay what the rarefaftion really 

 was ; it might be fix times lefs ; but it could not, I think, 

 be known by any gage, what it was, as I am perfaaded, that 

 the difierence of altitude between my baroin^'-gage, and that 

 of the moft perfe6l baromr, would at that time be impercep- 

 tible to the eye : it may be greater, for a quantity or (Iratum 

 of ele(5lric matter ^ of an inch in thicknefs, as in my tube, 

 may be invifiblc, when one of the fime denfity, but many 



3 C 2 miles 



* Perhaps becaufe denfer air, afFonliiig too much rcfiilance to its pa/llige, i. e. 

 ^ing a worfe conduftor, it takes a circuit by the rarer air in the lii^her regionr;, 

 from the place where it is pofitive. to tliat where negative. 



