434 



Prof. F. Elgar. On the Variation of [Mar. 13, 



hornblende, but the angles of cleavage (which, however, is in no case 

 very well defined) and of extinction make it more probable that the 

 mineral is augite. There are two or three crystals of a slightly 

 greenish colour, which show the characteristic form and cleavage of 

 augite, and one which in all respects better agrees with hypersthene. 

 The ground-mass consists of fairly numerous, lath-shaped crystallites 

 of a plagioclastic felspar, prisms of augite (?), often darkened with 

 f errite, granules of opacite and f errite, and possibly in some cases flakes 

 of mica. These are scattered in what may be a glassy base, but it 

 is so densely crowded with extremely minute acicular crystallites 

 (colourless, probably felspar), and with a minute dust (ferrite in the 

 browner streaks, opacite in the darker) that, as the slide is rather 

 thicker than usual, I cannot be quite sure. Although, to the unaided 

 eye, and even when examined with low powers, this rock appears to 

 differ considerably from (5) and (8), yet with high powers the resem- 

 blance becomes much closer, so that we may, I think, confidently refer 

 it to the same group, and regard it as merely a more slaggy variety. 



It follows, then, from the above examination that the rocks which 

 form the actual peak of Antisana are augite-andesites, containing 

 at any rate occasionally hypersthene, and to the same group belongs, 

 though perhaps it is slightly more basic, the rock of the great lava 

 stream which has descended to Antisanilla, while the pitchstones of 

 Quebrada de Urcucuy must be representatives of a group with a higher 

 percentage of silica, i.e., rhyolites or dacites, probably the former. 



IV. " The Variation of Stability with Draught of Water in 

 Ships." By F. Elgar, Professor of Naval Architecture in 

 the University of Glasgow. Communicated by Professor Sir 

 William Thomson, F.R.S. Received March 6, 1884. 



(Abstract.) 



Of all the properties possessed by a ship none is more vital to her 

 safety and efficiency than that of stability. At the same time none is 

 dependent for its existence and amount upon so many or such diverse 

 and variable circumstances as it. The stability of a ship, both as 

 regards moment and range, is affected not only by the position of her 

 centre of gravity, which largely depends upon stowage, but also by 

 draught of water. If the centre of gravity be kept fixed in position 

 at various draughts of water, the stability will still vary very consider- 

 ably with the draught, and often in a manner that contains elements 

 of danger. 



The usual practice in investigating a ship's stability is to calculate a 

 curve of metacentres, and one or more curves of stability at certain 



