119 



Birnor (1881) oxporimoiittHl on the amount of water needed by 

 potatoes. Four series of experiments were made, eaeh inchidin<; five 

 tubs having: dirt'erent amounts of water, as shown in the foUowing 

 table, which mves the average of the four series: 



These figures show a steady increase in the amount of harvest with 

 increasing moisture. 



The student will notice that in these experiments where the plants 

 are kept in tubs under protection from natural rains the watering and 

 growth go on under continued sunshine. The experiments therefore 

 correspond with the case of irrigation in a dry, sunny climate, and it 

 is not to be understood that the same amount of water deposited 

 naturally by clouds, with attendant long-continued obstruction of 

 sunlight and heat by the clouds, would have produced the same large 

 crops. 



R. Heinrich (ISTG) experimented at Mecklenburg on the influence 

 of water on grasses and clover. Ten sets of tubs filled with sterile 

 i^^and were sown with grasses and clover and watered daily, with 

 results as shown in the following table : 



This shows that the harvest increased steadily up to 8 millimeters 

 of rainfall daily, but for 9 or 10 millimeters per day the increase in 

 harvest was so slight that we may consider 9 millimeters, with an 

 average harvest of 162 grams, as the best that could be obtained under 

 the temperature and sunshine prevailing that year at Mecklenbui-g. 

 Doubtless a differwit quantity of water would be required in order to 

 obtain the maximum harvest in other climates. 



