1867.] 



Observations among the Alps, 



323 



Faulhorn it was 1*345, a value much higher than any which was obtained 

 at any time on Mont Blanc, or seemed likely to have been obtained at that 

 hour had the observations been continued so long. "What share the greater 

 depression of the lower station in the experiments of 1832, the more com- 

 plete isolation of the upper station^in those of 1866, or variable atmospheric 

 conditions in both sets may severally have had in contributing to this effect, 

 remains a matter for future investigation. The respective heights of the 

 stations are as follows : — 



English ft. English ft. 



^^"1*'°™ S^^n Difference 6853 

 Brienz 1946 J 



Mont Blanc .... 15784 1 ^^^^^^^^ j^g^g 



Chamonix 3425 J 



Professor Forbes gives the numbers — 



^^!^^^^^° ^^^l] Difference 6844. 



Brienz 1903 J 



The sky during the observations was not only cloudless, but, as seen from 

 the summit, remarkably clear. 



The observations have all been reduced by means of Tables derived from 

 Gmelin's 'Chemistry,' vol. i. p. 231, to what they would have been had 

 the mean temperature of the liquid during each minute been 32° F. 



By a prolonged and careful comparison of actinometers (K) and (A), 

 the factor for reducing the indications of (K) to the standard of (A) was 

 found to be 1-29. 



Considerable practice is necessary to acquire expertness in the use of the 

 actinometer employed. It is desirable, as nearly as may be, to work it at 

 such a temperature that the rise in the sun may be equal to the fall in the 

 shade. If the mean of the two mean temperatures of the liquid, in taking 

 the shade observations which precede and follow a given sun, differ much 

 from the mean temperature of the liquid during that sun, a sensible error 

 will be introduced. This, however, is to a great extent eliminated by taking 

 the mean of three, and still more completely by taking the mean of five 

 successive actinic results in column (I). 



The difficulty of using the instrument was overcome by the kind coope- 

 ration of several friends for the Chamonix observations. To the good 

 offices of my cousin, Mr. G. F. Hodgkinson, were added those of a lady, a 

 worthy sister of one of the foremost mathematicians of his year, and her two 

 nieces. Under her auspices an admirable arrangement of the work was 

 made, by which each of the party was responsible for a precise and definite 

 function, the adjustment and direction of the instrument, with the shading 

 and unshading, the watch, the readings, and the records. To this friendly 

 and efficient help I am greatly indebted for whatever success has been 

 achieved. How small this is, no one can be more sensible than myself ; 

 yet I venture to hope that when the difficulty of the undertaking is con- 

 sidered, to those at least who are acquainted with the experience of Prin- 

 cipal Forbes in 1832, 1841, and 1842, as given in his Bakeriaa Lecture, 



