the Depth of the Mines in the Hartz. 403 
lometimes the column breaks beneath the fummit, and 
there remain only fome lines of the quickfilver fuf- 
.pended to the top. 
It is for barometers cleared of air to this degree that 
my formula have been determined : and I had the fatis- 
faftion to find that they anfwered in the Hartz juft as 
they had done upon the mountains in the neighbourhood 
of Geneva, where they took their origin. 
Another remarkable circumftance which relates to the 
Barometer itfelf is as follows. Having occafion for cor- 
refponding obfervations in fome places of my rout, I ap- 
plied to obfervers who had good Barometers; amongft 
which I met with one of Mr. dollond’s. Thefe Baro- 
meters I compared with my own, being well allured 
beforehand, that I Ihould find a difference in the heights 
indicated, from the circumftance of their having cifterns 
at 
fecoufle. Quelquefois meme la colomne fe romp au deffous du fommet, et il 
ne refte que quelques lignes de mercure fufpendu. 
C’efl pour des Barometres purges d’air a ce point, que mes formul.es ont ete 
determinees: auffi ont-elles rtuffi dans le Hartz, t ut comme dans ies mon- 
tagnes des environs de Geneve, ou elles ont pris nalffance. 
Void encore, Monfieur, une circonftance remarquabie, qui regarde le Baro- 
nijtre meme. Ayanteu befoin en quelques endioits de raa route, d’obfervadons 
correfpondentes, je m’adreflai a des amateurs, qui avoient de bons Barometres. 
J’en trouvai un encr’ autres de M. dollond. Je comparai ces Barometres au 
mien, etant bien alTure de trouver de la difference dans la hauteur indiquee ; 
parcequ’ils avoient des refervoirs en bas; ce qui fait que la colomne barome- 
G g g 2 triquc 
