54 Prof. H. H. Dixon. [Nov. 7, 



branch at its middle length, was selected. The diameter of the stem was 

 0*85 cm. and T05 cm. at each end respectively. Its wood was also laid bare 

 by an oval scar where another lateral branch was removed. This scar was 

 1*3 cm. by 1*1 cm. The small lateral branch was cut to a length of 10*5 cm., 

 and its wood had a diameter of 0*47 cm. Not more than a quarter of its cross- 

 section was in a state suitable for transmitting water — the greater part being 

 occupied by b]ackened duramen. This lateral branch was fixed water-tight in 

 a rubber bung in an orifice in the bottom of a tank in such a manner that the 

 cut end of the lateral branch projected from the tank, while the supporting 

 stem was immersed in the water in the tank. With the head of 30 cm. 

 - 300 gramme were transmitted per hour. The transmitting cross-section 

 was about 0'043 sq. cm. One-quarter of this area would be lumen : therefore 

 the velocity of flow must have been 27'2 cm. per hour. If we reduce this to 

 unit head the velocity becomes about equal to 9*4 cm. per hour, which again 

 falls within the limits of the results obtained by the previous method. 



Throughout this discussion it has been assumed that the velocity is 

 proportional to the pressure. This is usually done. Fig. 3 shows that this is 

 certainly approximately true for pressures up to 16 units head. On fig. 3 

 are plotted the results of many experiments, the object of which was to 

 determine if the amount transmitted in a given time is proportional to the 

 pressure, as the former assumption would involve. The utmost care was 

 taken to obtain clean water to supply to the wood and to free the latter from 

 clogging material as much as possible. The water used was carefully 

 distilled, the vessels and tubes of supply were repeatedly washed with 

 distilled water before using, and the surface of the wood freshly cut and 

 rinsed with distilled water immediately before the experiment began. The 

 woody cylinder was 3 cm. long. It was supported horizontally, and a tongue 

 of bibulous paper w 7 as applied to the outer surface of the wood and hung down 

 vertically into a little phial which received the drops of transmitted water. 

 The time required for five drops to fall from the tongue was noted by means 

 of a stop-watch, and the weight of these five drops determined by weighing 

 the phial. The mean of three such observations is given below (Table V) for 

 each pressure. 



These results confirm the received view that the amount transmitted varies 

 directly with the pressure. The slight bending over of the line joining the 

 observations at the high pressures is amply explained by the gradual clogging 

 which takes place despite all precautions ; for the experiments giving these 

 results were made with the same piece of wood after those at lower pressures 

 had been carried out. 



