Observations among the Alps, 307 



Professor Forbes gives the numbers — 



English ft. English ft. 



Faulhorn 87471 



Brienz 1903 ) 



Difference 6844. 



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 tempe- 

 ratures 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 suc- 

 cessive actinic results in column (1). 



The difficulty of using the instrument was overcome by the kind 

 cooperation 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 

 Principal Forbes in 1832, 1841, and 1842, as given in his Bakerian 

 Lecture, the results will not appear either disappointing or discou- 

 raging. The season was extremely unfavourable for the further pro- 

 secution of the work. Looking to the imperfection of the instrument 

 employed by Principal Forbes in his observations in 1832, it would 

 seem to be highly desirable that his experiments on the column of 

 air between the Faulhorn and Brienz should be repeated, and that 

 other pairs of stations, intermediate in character to that and the 

 Chamonix pair, should be essayed. I have selected, in the hope of 

 future opportunities, the following among others : — 



X2 



