VOL. XXXVI.] PHILOSOPHICAL TRANSACTIONS. 3Q3 



in the pocket, and much less managed and understood by persons not used to 

 experiments, Mr. Clarke was resolved to perfect the hydrometer for the use of 

 those that deal in brandies and spirits, that by the use of the instrument they 

 may, by inspection, and without trouble, know whether a spirituous liquor be 

 proof, above proof, or under proof, and exactly how much above or under : 

 and this must be of great use to the officers of the customs, who examine im- 

 ported or exported liquors. 



After liaving made several fruitless trials with ivory, because it imbibes spi- 

 rituous liquors, and thereby alters its gravity, he at last made a copper hydro- 

 meter, represented by fig. g, having a brass wire of about -f inch thick going 

 through, and soldered into the hollow copper ball, Bb. The upper ball of this 

 wire is filed flat on one side, for the stem of the hydrometer, with a mark at 

 m, to which it sinks exactly in proof spirits. There are two other marks, a 

 and B, at top and bottom of the stem, to show whether the liquor be -^ above 

 proof, as when it sinks to a, or -pL. under proof, as when it emerges to b, when 

 a brass weight, such as c, has been screwed on, to the bottom at c. There 

 are a great many such weights of different sizes, and marked to be screwed on, 

 instead of c, for liquors that differ more than -jV from proof, so as to serve 

 for the specific gravities in all such proportions as relate to the mixture of 

 spirituous liquors, in all the variety made use of in trade. There are also other 

 balls for showing the specific gravities quite to common water, which makes 

 the instrument perfect in its kind. 



u4n unusual Aurora Borealis seen at Geneva. By Mr. G. Cramer* Prof. 

 Math, there. N° 413, p. 279- 



This phenomenon occurred Feb. 15, 1730, n. s. And what was chiefly 

 remarkable in it, was a large meridional zone, like a rainbow in its figure, but 

 broader. It was terminated by two parallel arches. The superior insisted with 

 one side on the true point of east, and with the other on the point of south- 

 west, or west-south-west: whence its middle declined about 15° from south to 

 east, and was diametrically opposed to the middle of the aurora borealis. Its 



* Gabriel Cramer was born at Geneva in I/O*, and was appointed Professor of Mathematics 

 there at 19 years of age. The public are indebted to him for the edition of the works both of James 

 and John Bernoulli, which he collected and published. In 1746 he finished his L' Analyse des 

 Lignes Courbes, •ito. but which was not published till 1750. Cramer was a member of several 

 academies; as those of London, Berlin, Montpelier, Lyons, Bologna, &c. He was celebrated not 

 only for his mathematical talents, but as a universal genius, a kind of living Encyclopedia, and a 

 man of most exemplary conduct. He died 1752 in Languedoc, where he had gone for the 

 recovery of his health. 



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