353 Prof. Tyndall on Foam and Hail. 



to KR, meeting OP in V. Then 



0F = 2.0U, OV=i.OD. 



Draw OQ, OS parallel to KF, KR respectively. Then 



sin (POQ- sin POR) sin POS = i sin (POS-POR) sin POQ. 



15. POS is given in terms of POQ, POR, and the indices of 

 the four edges by 



z(cot POS- cot POR) = cot POQ- cot POR ; 

 or 



i sin POQ tan 6=. sin (POR- POQ), 



tan (POS-iPOR) = tan i POR tan(| -^Y 



These expressions are of use in calculating the angles between 

 the faces of crystals, and also in constructing models, and draw- 

 ing figures of crystals. 



XLIX. Remarks on Foam and Hail. By Professor Tyndall. 



ON Monday, the 13th of April, I walked from Ventnor to 

 Freshwater Gate along the southern coast of the Isle of 

 Wight, The day was stormy, and the sea consequently high. 

 Owing to the retardation of their inferior portions by friction, the 

 waves, as they approached the shore, became steeper, the crests 

 advanced, and finally fell over like cataracts. Sometimes three 

 or four such waves were observed following each other and 

 charging with great impetuosity up the beach, bounding over 

 isolated rocks which, when the wave receded, stood six or eight 

 feet above the water. Sometimes two wave;^ intersected each 

 other, and at the point of junction, as both fell forward, a dark 

 cavernous space, somewhat resembling those observed at the ends 

 of glaciers, existed for an instant. In the next moment the air 

 within it was whipped into foam. The production of a great 

 quantity of foam was indeed the consequence of the commotion. 

 Large masses of it, which shook like elastic jelly in the wind, 

 were collected wherever it could find shelter, from which pieces 

 were sometimes detached and blown like gigantic snow-flakes 

 inland. The remark I have to make here has reference to the 

 temperature of this foam. On passing a nook where a quantity 

 of it had collected, I dipped my hand into it and found it blood- 

 warm ; its taste, moreover, was intensely bitter. On examining 

 the water of the sea from which the foam had been produced, I 

 found it very cold. In fact it was a raw sleety day, and the 

 temperature of the sea-water was what might be expected in 

 such weather. The warmth of the foam appeared to be due to 



