26 



Prof. G. H. Darwin. 



[Nov. 22, 



fro relatively to the ascending column. The ink continues to spread 

 ont laterally and begins to fall on each side. In this stage if the ink 

 is not thick it is often very like a palm- tree, and for the sake of a 

 name T call this appearance an ink tree. The branches (as it were) 

 then fall on each side, and the appearance becomes like that of a 

 beech-tree, or sometimes of an umbrella. The branches reach the 

 ground, and then creep inwards towards the stem, and the ink, which 

 formed the branches, is sometimes seen ascending again in a wavy 

 stream parallel to the stem. 



Perhaps a dozen or twenty oscillations are requisite for making 

 the ink go through the changes from the first growth of the tree. 



The descending column of a pair of trees comes down on to the top 

 of the mushroom. I have occasionally, when the oscillations are 

 allowed to die, seen both tree and mushroom, but the successful 

 manufacture of the tree necessitates an oscillation of sufficient 

 violence to render the observation of the mushroom very difficult. 



The alternate thickening and thinning of the ink on the crests 

 seems to render it probable that with moderate oscillation the mush- 

 room vortices are still in existence, or at any rate that alternately one 

 and the other is there. With violent oscillation, when the stem of 

 the tree is much convoluted, as described below, it cannot be asserted 

 that the mushroom vortices exist, and I am somewhat inclined to 

 believe them to be then evanescent. 



Each side of the ink tree is clearly a vortex, and the stem is the 

 dividing line between a pair, along which each vortex contributes its 

 share to the ascending column of fluid. The vortex in half the tree 

 is clearly in the first place generated by friction of the vortex in its cor- 

 related mushroom, and is of course endued with the opposite rotation. 

 The ascending stem of the tree is a swift current, but over the mush- 

 room the descending current is slow until close to the mushroom, when 

 it is seen to be impelled by pulses. 



I was on one occasion fortunate enough to observe a mote in the 

 water which was floating nearly in the centre of a tree vortex, and 

 counted twelve revolutions which it made before it was caught away 

 from its fortunate position. 



If the adjoining crests are of unequal height the stem of the tree 

 is thrown over sideways away from the higher crest; and indeed it 

 requires care to make the growth quite straight. The ink in the 

 stem ascends with a series of pulses, and it is clear that there is a 

 pumping action going on which renders the motion of each vortex 

 somewhat intermittent, the two halves of. the tree being pumped 

 alternately. 



The amount of curvature in the stem of the tree depends on the 

 amplitude of the oscillation of the water. Figs. 4, 5, 6, give fair 

 representations of ink trees. , 



