238 



The sum total of daily maximum sun temperatures at Giessen is 

 much more nearly constant. 



In the Zeitschrift for 1882 Hoffmann gives the sums of the daily 

 positive readings of his naked bright-bulb mercurial thermometer in 

 the full sunshine; he also gives the sums of the temperature in the 

 shade, and computes the average discrepancy or probable error of 

 these numbers as deduced from their internal agreement year by year. 

 He finds the jjrobable uncertainty of the sums of maxima to be plus 

 or minus 1 per cent and of the sums of shade temperatures to be plus 

 or minus 10 per cent. These latter sums relate to low-lying stations, 

 such as Vienna and Dorpat, and these discrepancies diminish very 

 much when we consider high mountain stations, where the shade 

 temperatures of course give much smaller sum totals. He recognizes 

 that the advantage of using the shade temperatures lies in the greater 

 comparability of the observations made at different stations and with 

 different instruments, but that the sunshine method is also greatly 

 improved if the thermometers are perfectly similar and properly 

 compared together, as in the instruments made by Doctor Ziegler at 

 Frankfort. (8ee the report of the Senckenburg Association, 1879- 

 1880, p. 337.) Hoffman's observations with a varietj^ of instruments 

 convinced him that this difficulty as to instruments and exposures 

 is not insurmountable. He collects comparative readings at several 

 places and shows that the difference between the average tempera- 

 tures in the sun and in the shade is larger at higher altitudes; thus 

 at Giessen the average difference in summer at midday is 5° Reaum., 

 and the whole range of the differences between sunshine and shade 

 is from 3° to 15° Reaum. The corresponding average in the Hochge- 

 birge, 7,000 feet, is never less than 8° Reaum. At the Bernina 

 hospice, 8,113 feet, it is 25° Reaum. The average temperature of 

 these mountain stations is 16.4° Reaum., corresponding to an elevation 

 of about 6,000 feet. Similarly, J. D. Hooker observing a black-bulb 

 thermometer in the sunshine in the Himalayas, found a difference of 

 — 15° Reaum. at 7,400 feet elevation, as contrasted with 4.4° at sea 

 level. R. S. Ball, also using a black bulb, finds a difference of 18° or 

 20^ Reaum. in the Hochgebirge and of only 3° at Chiswick. 



These differences show the effect of the great dryness and mechan- 

 ical purity of the air in the Hochgebirge. Hoffmann considers the 

 smoke and clouds above us as affecting the difference between the sun 

 and shade thermometers, but says nothing of the earth's surface which 

 completes the " inclosure " of the thermometer. 



The date from which Hoffmann begins his summation for Giessen 

 is January 1 ; but as it would seem more proper to begin with some 

 definite phase of vegetation, therefore he investigates the accuracy 

 with which we can determine the initial phase and the effect of errors 

 therein upon the ultimate sums. By painting the buds of certain 



