336 Mr F. Darwin and Mr R W. Phillips, , [Nov. 23, 



20°/ o , so that we are not inclined to trust observations where the 

 reading of the stop-watch is less than 3". 



A second method of testing the apparatus was that of weighing 

 the quantity of water flowing from the siphon in given times ; the 

 siphon was allowed to run for 15 minutes, the potometer being 

 read at intervals of about a minute. The current was then 

 quickened and the amount of water yielded in 15 m. again weighed, 

 the potometer readings having been again taken. 



In the following table, column A gives numbers proportional to 

 reciprocals of the mean of the reading of the stop-watch taken 

 during the flow of the siphon at one pressure. Those in B are 

 proportional to the weights of water flowing from the siphon in a 

 quarter of an hour at three different pressures. (The actual 

 pressures are not given.) 



A B error 



2 < /o 



4-4°/ 



/ o 



If we call B the actual outflow from the siphon, and A the 

 outflow estimated by the potometer, we shall see that the error 

 does not reach 5°/ . The reading of the stop-watch corresponding 

 to the amount 408 in column A was 5 - 2". If the calculated flow 

 had been equal to the observed flow the reading would have been 

 4"9", and since in our experiments we should not base any conclu- 

 sions on a difference of 0'3", it is clear that the amount of dis- 

 crepancy between the observed and the calculated outflow is not of 

 an amount to influence the trustworthiness of our conclusions. 



But the best proof of the trustworthiness of the apparatus was 

 obtained by comparing the readings of the potometer, obtained 

 from a transpiring branch placed under varying conditions of 

 atmospheric humidity, with the readings of a psychrometer. 

 It is known that the amount of water absorbed by a cut branch 

 varies inversely with the variations in the relative humidity of 

 the atmosphere when the temperature remains constant*. It is 

 clear therefore that the readings of the potometer should bear 

 a certain ratio to those of the wet and dry bulb thermometer. 

 The plant used was a branch of Portugal laurel, the twigs being 

 somewhat altered in position by tying so as to give the whole 

 a convenient form for being placed under a bell-jar. The branch 

 was passed through a cork fitting into a horizontal plate of glass 

 on which the bell-glass could be placed so that the plant and the 

 thermometer could be exposed to a moist atmosphere, the humidity 



* C. Eder, " Untersuchungen iibev die Ausscheidung von Wasserdampf bei den 

 Tflanzen." Sitz. der k. Akad. drr Wiss. 3. Wien, Bd. 72, Oct. 1875. 



