Zach on Repeating Circles. 353 



slightly impregnated with sulphate of iron, and had a trace of 

 muriatic acid ; and that from the latter mine contained a very 

 minute portion of the muriate of lime." 



Mr. Fox terminates this paper with the following additional 

 notice : — 



" Since my last communication on the subject of the tempe- 

 rature of mines, I have had a thermometer, four feet long, 

 placed in a hole three feet deep, in a copper vein, at the end of 

 the deepest level, or gallery, in Dolcoath, which is 230 fathoms, 

 or 1 380 feet, under the surface : a spot where no workmen were 

 employed, and where the current of air must have been small. 

 The hole was filled with clay round the stem of the thermometer, 

 so as to prevent the circulation of air near the bulb, and in this 

 situation it remained more than eight months. It was often 

 examined during that period, and was always found to indicate 

 a temperature of 75°, or 75^°, unless it had recently been over- 

 flowed by water. This happened several times, in consequence 

 of accidents to the machinery of the mine, and more than once 

 the water filled the level for some weeks. As soon as it had 

 subsided, so as to permit access to the thermometer, the quick- 

 silver was observed to have risen to 77°, but in two or three 

 days it again fell to 755°." 



LXXIII. On Repeating Circles. By the Baroji de Zach.* 



Genoa, January 1, 1823, 

 "Vl^HEN M. Schwerd said in his work entitled La petite 

 ^~ Base de Spire, 8j~c. which we noticed in page 201 of the 

 7th volume of this Correspondance, " that according to the opi- 

 nion of astronomers, the zenith distance of a star, observed "with 

 the best repeating circles, cotdd not, even after many thousand 

 repetitions, be determined and depended upon to a second ;" we 

 added in a note, " We may affirm yet further, that this uncer- 

 tainty extends to fotir seconds." 



We did not exaggerate in this assertion ; on the contrary, 

 these differences in the zenith distances, observed with the best 

 repeating circles, have more and more exceeded the limit we 

 had assigned to them. This has been the case in the most deli- 

 cate operations it was possible to undertake ; those in which 

 the greatest care and the utmost precautions were necessarily 

 employed. The most important and exj^nsive ojieration of 

 this kind which has been undertaken, is that which was exe- 

 cuted in France in 1 792, and two following years, for measuring 

 an arc of the meridian from Dunkirk to the Balearic islands. 



* From Zach's Cnrrcspimdnnce Aslronowi'jiir, vol. viii, i)age 3. 

 Vol. 61. No. 301. Mffj/ 1823. Yy Both 



