182 PHILOSOPHICAL TRANSACTIONS. [aNNO 1777. 



Height of tlie entry of the mine, called Alte Seegen, above a certain point in tlie valley of Bremeke 

 1 0'2. 18 French toises; height of the entry of the same mine, above another point near Lasfelde in 

 the valley of Osterode, 173.81. 



One of these barometrical measures, taken in open air, was found to agree very 

 nearly with the geometrical measure; and the other dilfered only li toise in excess. 



A twisted brass wire 5 toises long, 2 puncheons, a semicircle, and a compass, 

 are all the instruments made use of by the subterraneous geometer. By means 

 of the '2 puncheons, he extends his wire in the direction of the way which he is 

 measuring; and by practice he acquires a habit of always stretching it to the same 

 degree. His semicircle, which is very light, being suspended at the middle of 

 the wire, shows him its inclination. By this means he has a right-angled tri- 

 angle, of which the hypothenuse and angle at the base are known. He has 

 consequently the vertical height and horizontal distance gone over. After this 

 he suspends his compass to the wire, to find out its declination, and consequently 

 the direction of his horizontal line. It is in this manner that he draws the plan 

 and section o*" these subterraneous labyrinths. It is likewise by this means that 

 he goes over hills and vales, in order to determine points corresponding to his 

 pits and galleries. But is this a method that may safely be depended on? The 

 fact answers, and saves the trouble of long reasonings. A miner, solely on the 

 faith of his geometer, and in the absolute obscurity of the entrails of the earth, 

 undertakes a labour that is to cost him years, in daily boring through a rock. 

 Another miner sets out to meet him from some other mine, or from without. 

 At the end of a determined measure, the gnomes begin to hear each other, and 

 at length they meet. Having observed some of these points of rencounter in 

 the galleries; it is sometimes difficult to perceive the small winding which has 

 been necessary for their meeting end to end. 



Mr. de L. communicates some other barometrical observations, not verified by 

 geometrical survey ; by which he determined the height of some points of the 

 Hartz relatively to the plain, and chiefly the highest point. This greatest eleva- 

 tion, called the Blocksbcrg or Brocken, is situated in the estate of Count de 

 Verniguerode. And the result of the observations was as follows, viz. 



The small hut, at the summit of the Brocken, above Oder-brucl<e, 17'i.93 French toises; Oder- 

 brucke above Claustlial, 91.39; Clausthal above Gottingen, '2 10. '21; Gottingen above Hanover, 

 i6A5; total elevation of tlie Brocken above Hanover, ;>30.y8. 



It will be easy to come at the elevation of Hanover above the level of the sea, 

 m order to complete this measurement; corresponding observations of the baro- 

 meter will be sufficient for that purpose. In the mean time it is easy to estimate, 

 either by the barometer itself (the mean height of which during the month of 

 October was 30.1 English inches on a 'id floor) or by the small declivity of the 

 rivers between Hanover and the sea, that the elevation ut Hanover above that 

 level is not very considerable. 



