54 Transactions of the South African Philosophical Society. 



Humidity. 



No. of Days. 



8-inch 

 Copper Pan. 



Screened Tub. 



Piche Tube. 







Inches. 



Inches. 



Inches. 



Under 30 % 



8 



•515 



•403 



•49 



30-40 



75 



•400 



•291 



•37 



40-50 



79 



•303 



•203 



•27 



50-60 



88 



•205 



•134 



•20 



60-70 



54 



•160 



•104 



•14 



70-80 



42 



•114 



•069 



•10 



80-100 



19 



•053 



•036 



•05 



The above formula applies to these results fairly well also, the 

 numerical coefficient becoming respectively 10,500,- 14,000, and 11,000 

 approximately. But the value of e is uncertain in the pan results 

 because the evaporation from that source is greatly interfered w^ith 

 by rain when the air is very damp. It should be remarked also i\i^ 

 wiiile the humidity refers to the mean of hourly observations taken 

 during the civil day, and the tank evaporation to civil day totals, the 

 evaporation from the other gauges is measured at XX., and therefore 

 the day begins four hours earlier for the latter than for the humidity. 



In comparing the evaporation from the tank with the temperature 

 of the air it has been tacitly assumed that the temperature of the 

 water would follow that of the air pretty closely, and hence that 

 for the one given w^e could roughly read the other. Now it was 

 Symons's contention that a small gauge becoming heated to a very 

 much greater degree by the sun's rays, thereby, for that chief 

 reason, lost more than a large gauge. Hitherto I have not been 

 able to make regular routine observations of the temperature of the 

 water in my several gauges ; but the following table giving some 

 casual measurements during the winter and spring of 1901 has some 

 points of interest. It contains (1) the hour of observation, (2) the 

 temperature of the tank, (3) of the screened tub, (4) of a number of 

 5-inch copper pans, and (5) of the air : — 



Hour. 



Tank. 



XXI. 



55 4° 



XI. 



572 



XXIII. 



51^8 



XI. 



55-5 



XXIII. 



55-0 



XL 



60^6 



XL 



65^4 



XL 



58-8 



XL 



67-2 



XXIL 



70^8 



XXIII. 



69-0 



Screened 

 Tub. 



58.5° 



51 • 4 



53-9 



46^2 



58^4 



55-6 



64^8 



47^9 



57^5 



68-6 



66^5 



Copper Pans. 

 Mean. 



60-9° 

 48^5 



54 2 

 54-8 

 64^4 

 74^8 



55 2 

 68-6 



Air. 



69-2' 

 50^0 

 57-0 

 53-0 

 70^3 

 82^0 

 56-1 

 ()8^0 

 65^4 

 60-0 



(Strong Wind) 



(Cloudy) 

 (Do.) 



