Mr. Forbes on the Geographical Position of Chamouni, 8fc. 61 



If, therefore, the change first effected does not give a result 

 nearer to zero than R, the second one will, and vice versa ; 

 contrary to hypothesis. 



Hence the proposition must be true. 

 Cambridge, Dec. 20, 1832. 



XII. Notice respecting the Determination of the Geographical 

 Positions of the Village of Chamouni, and the Convent of the 

 Grand St. Bernard. By J. D. Forbes, Esq. F.R.S. L. # E* 



AMONG the instruments with which I provided myself 

 before leaving Britain, was a portable altitude and azi- 

 muth circle, constructed for me upon Captain Kater's prin- 

 ciple, by Mr. Robinson, precisely similar to that which ac- 

 companied Captain Parry upon his last journey towards the 

 Pole. It is considerably larger than the instruments usually 

 made by Mr. Robinson of this form, and proportionably more 

 perfect. The circles have a diameter of 4| inches, and are 

 divided to 15". The azimuth circle has three verniers; that 

 for altitude, two. Two telescopes accompany the instrument, 

 with three simple eye-pieces, and a diagonal one. The level 

 is an excellent one, and there are various other appendages 

 which I shall not at present particularize. 



I intended to use this circle rather as a theodolite than an 

 astronomical instrument ; but my friend Professor Gautier, of 

 Geneva, to whose kindness I have been much indebted during 

 my stay in that place, suggested that I should make it the 

 companion of my journey in Savoy, and determine the posi- 

 tion of two^ points of interest, (for neither of which have we 

 any direct observations,) Chamouni, and the Convent of the 

 Grand St. Bernard ; the last in particular is interesting, from 

 the importance which attaches to it as a meteorological sta- 

 tion. I readily adopted his suggestion ; and had reason to be 

 satisfied with the capability of the instrument to resist shocks, 

 having been carried during the greater part of the journey 

 upon a mule. The weather at Chamouni was far from fa- 

 vourable, and I only obtained two series of simple altitudes 

 of the sun on different days, for the time, and three altitudes 

 of Polaris at intervals during a cloudy evening. At St. Ber- 

 nard I was more fortunate ; I obtained corresponding altitudes 

 on the evening of the 30th of August, and the succeeding 

 morning, which gave the time of apparent midnight: and I 

 took eight successive altitudes of Polaris, half with the face of 



* Read before the Royal Society of Edinburgh, December 3, 1832 ; and 

 communicated by the Author. 



