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ges to B) when aBrafsWeightjfuch as C, has been fcrew* * 
ed on, to the Bottom at c. There are a great many 
fuch Weights of different Sizes, and marked to be. 
fcrewed oh, inftead of C, for Liquors that differ more 
than A from Proof, fo as to ferve for the fpecifick Gra- 
vities in all fuch^ Proportions as relate to the Mixture 
of fpirttuous Liquors, in all ‘the Variety made Ufe of 
in Trade. There are alfo other Balls for (hewing the 
fpecifick Gravities quite to common Water, v/hich 
makes the Inftrument perfed in its Kind. 
VII. An Account of an Aurora Borealis attend^ 
ed with unufual Appearances^ in a Letter frofn 
the Learned Mr. G. Cramer, 'Pro/. Math. 
Genev. to James Jurin, M T>. and F. ^ S. 
I Have been fo overcharged with Buhnefs (ince I 
came here, that I had hardly Time enough to think 
of writing. 
Being now a little more at Leifure, I would not 
mifs the Occafion of an Aurora Borealis^ which ap- 
peared here the 15th of Feh. N.S. accompanied with 
fome Circumflances rare enough to be worth your Con- 
fideration. , , , . 
• The Aurora it felf had nothing extraordinary ; it 
was a quiet one, that is, without any fenfible Motion, 
except, perhaps, an alternative Increafe and Diminution 
of apparent Altitude. Whether it was for this Rea- 
fon, or becaufe the Light had its Edge imperceptibly 
confounded with the Colour of Heaven, feveral Peo- 
ple judged of that Altitude feverally. There are fome 
I 
