[ 384 ] 



efpecially, as from the difficulty of making a perfedl vacttum 

 in the ftandard-barom'', and of comparing the altitude of the 

 merc>' which ought to be in it, with that in the gage, I found 

 the exhibition of the genuine Aurora Borealis, the mod cer- 

 tain mark of great rarefadion, as not I'cquiring a comparifon 

 of the barom' and pear gages. 



I CONNECTED with the rec' (which was ufed in the fore- 

 going trials of the pump's exhauiling power, and whofe capa- 

 city was 122 cubic inches) by means of a brafs box fixed on 

 the top of it, one end of a glafs-tube twenty-one inches long 

 and -^ of an inch in diameter, furnillied at one end with a 

 brafs cap terminating in an open pipe, which was infertcd la- 

 terally into the box of the rec : the other end of the tube 

 was clofed fometimes with a plate of brafs cemented on it, 

 and at other times with a glafs plate ; both which afforded 

 the fame appearances. The tube was extended horizontally, 

 being fupported at the end remote from the reC by a glafs- 

 pillar ; and it was exhaufted with the reC j during the procefs 

 of which, I applied to the tube in the dark, an eledlrified 

 glafs-tube, which produced the following appearances in re- 

 peated trials. 



The tube fhewed no eledlricity from prefenting to it the 

 excited tube, until the rarefaction was about 1 00 ; then there 

 appeared within it denfe and bright corufcations, but they 



were 



