Observations among the Alps. 305 



nomctrical observations made last summer on the summit of Mont 

 Blanc and at Chamonix, and at the same time to thank the Com- 

 mittee of the Royal Society for the grant which they were so good as 

 to vote me for that object. 



I reached Chamonix on the 7th of July, in bad weather, which had 

 been prevailing for some time, but which ushered in a fine week very 

 opportunely for my work. After allowing a few days for the weather 

 to settle and for the snow to consolidate, I left Chamonix in the after- 

 noon of Friday the 1.3th for the Grands Mulcts, having previously 

 arranged for a corresponding series of observations being taken the 

 next morning in the valley. Leaving the Grands Mulets at about 

 2\ a.m. on the 14th, I reached the summit of Mont Blanc about 

 8 a.m., and proceeded at once to work. 



I had brought with me from England two of Newman's mountain- 

 barometers, a thermobarometer of Casella, six small thermometers 

 graduated on the stem (three for the dry-, and three for the wet- 

 bulb observations), three of the tubes described in Appendix (A), with 

 two of the actinometers in each. I carried besides an aneroid by 

 Cooke, which proved to be of excellent quality. The third set of 

 apparatus was taken in some faint hope that I might be able to ar- 

 range for a third set of simultaneous readings at the Grands Mulets. 

 In this I was disappointed. Notwithstanding the greatest care had 

 been taken, one of the barometers was found on the Brevent on the 

 9th to be deranged, and one of the actinometers to be broken ; and 

 on the 12th a second actinometer was broken at Chamonix by an ac- 

 cident. I thought it best to leave the remaining barometer for the 

 valley observations, and to depend upon the thermobarometer, as being 

 more portable and less liable to fracture, for the readings on the 

 summit. I was eventually obliged to rest satisfied with a single ob- 

 servation of this ; and the downward range of the small thermometer 

 unfortunately proved too limited for the wet-bulb readings. Thus 

 the meteorological observations at the upper station are of the scan- 

 tiest. Neither above nor below were the actinometrical readings so 

 continuous as I had wished to make them. I had no one with me on 

 the summit capable of rendering me the smallest assistance ; but it 

 is some consolation to think that, even had this been otherwise, the 

 results could not, under the circumstances, have been materially 

 enhanced. 



There either did not exist," or I failed to detect, as the sun's 

 altitude increased, anything like a uniform progression of actinic 

 power at either station during the limited time in which the obser- 

 vations were continued. 



The results do little more than determine the ratio of the average 

 intensity at the two stations for a portion of the forenoon. This 

 indeed was the main object which I had in view. For looking at the 

 experience of Principal Forbes under easier conditions, when the 

 continuance of the observations, as long as the clearness of the sky 

 might last, presented no difficulty, I did not at all anticipate being 

 able to trace a dependence of the actinic power on the hygrometric 

 Phil. Mag. S. 4. Vol. 33, No. 223. April 1867. X 



