3i38 On the Relation between the Height of Waves 



finally accumulate, may be arrested by the silica, and converted into 

 insoluble fluosilicates (fluorides of silicon and of a metal) ; and a 

 Bamboo, for example, secreting Tabasheer, may effect this change 

 where one less rich in silica cannot determine it. The slow or quick 

 drying of a stem may also affect the fixation of fluorides in the 

 stems or trunks of plants. 



The sources of the fluorine found in plants may be regarded as 

 pre-eminently two, — (1.) Simple fluorides, such as that of calcium, 

 which are soluble in water, and through this medium are carried 

 into the tissues of plants ; and (2.) Compounds of fluorides with 

 other salts, of which the most important is probably the combination 

 of phosphate of lime with fluoride of calcium. This occurs in the 

 mineral kingdom in apatite and phosphorite, and in the animal king- 

 dom in bones, shells, and corals, as well as in blood, milk, and 

 other fluids. 



The recent discovery of the author's communicated to the Royal 

 Society of Edinburgh, (page 349) has shewn that fluorides are much 

 more widely distributed than is generally imagined, and that the trap 

 rocks near Edinburgh, and in the neighbourhood of the Clyde, as 

 well as the granites of Aberdeenshire, and the ashes of coal contain 

 fluorides, so that the soils resulting from the disintegration of those 

 rocks cannot fail to possess fluorides also. All plants, accordingly, 

 may be expected to exhibit evidences of their presence, in the fol- 

 lowing portions of their tissues or fluids : — 



1 . In the ascending sap, simple fluorides. 



2. In the descending sap, in association with the albuminous 

 vegetable principles, and in the seeds or fruits, in a similar state of 

 association, fluorides along with phosphates. 



3. In the stems, especially when siliceous and hardened, fluorides 

 in combination with silica. The investigation is still in progress. 



Observations on the Relation between the Height of Waves 

 and their Distance from the Windward Shore ; in a Letter 

 to Professor Jameson. By THOMAS Stevenson, Esq., 

 F.R.S.E., Civil Engineer. 



Edinburgh, September 16, 1852. 



Dear Sir, — In designing a harbour or sea work, the en- 

 gineer, in order to avail himself of the advantage which is to 

 be derived from past experience, must endeavour, to the best 

 of his power, to institute a comparison between the given lo- 

 cality and some other which he supposes to be in pari casu. 

 Such a similarity, however, of locality and other physical 

 peculiarities is hardly, if ever attainable, and all that he can 



