180 PHILOSOPHICAL TRANSACTIONS. [aNNO 1777. 



account of the extreme hardness of the rock.; and others dug out from the sum- 

 mit of the hill, at the depth of 4 feet, lying in a rich, light, hlack mould, 

 which, after having been exposed to the air for some time, turned to a whitish 

 ash-colour. On the summit of this hill is a small plane, yu paces long, by 1^ 

 wide, surrounded by rocks, from 6 to 8 feet high, like a breast-work, extremely 

 craggy, and difficult of access on the outside, but rising from the centre, in the 

 inside, to the top of the breast-work, with a gentle slope of turf, equal to any 

 of the finest on the sheep downs.* 



XXI. New Electrical Experiments and Observations; with aii Improvement of 



Mr. Canton s Electrometer. By Mr. Tib. Cavallo. p. 388. 

 See Mr. Cavallo's Treatise on Electricity, vol.2, for this paper. 



XXII. Barometrical Observations on the Depth of the Mines in the Hartz. 

 By John Andreiv de Luc, F. R. S. p. 401. From the French. 



These mir ;s are extremely deep; and Mr. de L. was desirous to try in them 

 his rules for measuring heights by the barometer, that he might know whether 

 in those pits, where exhalations of all kinds spread themselves, the condensations 

 of the air follow the same laws that they do out of them. And he had the satis- 

 faction to find that they answered in the Hartz, just as they had done on the 

 mountains in the neighbourhood of Geneva, where they took their origin. A 

 remarkable circumstance which relates to the barometer itself is as follows. 

 Having occasion for corresponding observations in some places of his route, he 

 applied to observers who had good barometers; among which was one of Mr. 

 Dollond's. These barometers he compared witii his own, being well assured be- 

 forehand that he should find a difference in the heights indicated, from the cir- 

 cumstance of their having cisterns at the bottom, which makes the barometrical 

 column always shorter in these than it is in a plain tube in the form of a siphon. 

 Accordingly this was the case in all these barometers; they all stood lower than 

 Mr. de Luc's, but varied from each other according to particular circumstances, 

 depending chiefly on the diameter of the tube, and the figure of the cistern. 



The first of his observations was made in 3 mines near Clausthal, called the 

 Dorothea, the Caroline, and the Benedict; the depths of which were found to 



be as follows, viz. 



The deptli of the Dorothy pit between two fixed points, 1 68 .96 French toises ; that of the Caroline, 

 relatively to the same point at die mouth, 170.74 ; tliat of tlie lowest searching gallery of tlie Benedict, 

 relatively to tlie same point, H-3.y() : all agreeing within 1 or 2 toises of the geometrical measureiiients. 



* I'he specimens sent witli tlie above letter to tlie iv. s., having been examined by st)me members 

 well acquainted with volcanic productions, were by tliem judged to be real lava ; and it was their 

 opinion, that if a groat quantity of the like substance be found on the hill from %\ hich these pieces 

 were taken, tlic hill most probably owes its origin to a volcano. .1. p. — Orig. 



*i 



