25 



putting a piece of dried protochloride, sesquichloride, or protosulphate 

 of iron, in a solution of silicate of potash : — 



3(2PeASi02) + 2(KO,C02). 



This would, in all prohability, be the silicate formed by the mutual 

 decomposition of an alkaline silicate and sulphate, or bicarbonate of 

 iron. 



The great facility with which this mineral decomposes and behaves 

 in acids, and its peculiarities generally, would seem to show that the 

 silicates of zinc and iron are in some sort of combination, and not simply 

 intermixed. If from the whole we deduct not merely the oxide of iron, 

 but also the amount of silica and water combined with it, the remainder 

 will contain oxide of zinc, silica, and water, in the proportions repre- 

 sented by the formula 2ZuO,Si02,HO. 



Perhaps many other minerals containing peroxide of iron, &c., would 

 present us with a like phenomenon, if we could dissolve one constituent 

 like the silicate of zinc. There are, no doubt, many cases where foreign 

 substances cannot be considered to be merely mechanically mixed in a 

 mineral, and yet cannot be held to replace some constituent isomorphi- 

 cally, which may be explained in this way. Indeed, it is probable, that 

 many of the so-called isomorphic replacements are in reality such com- 

 pounds, held by a very feeble affinity, but which, unlike- the one here 

 in question, cannot be dissected. 



TheREY. Samuel Hatj&htoi^, M.A., F.R.S., Fellow of Trinity College, 

 Dublin, read the following paper: — 



Ok a Geaphical Mode of Calculating the Tidal Deift of a Yessel 

 m the Ieish Sea oe English Channel. (Plate II.) 



The change of level in the surface of tidal water, between two given 

 hours, may be graphically calculated by the method given by Mr. Airy 

 in his Treatise on Tides and "Waves. Let a circle be described whose 

 radius is half the Range of Tide, and painted on a vertical wall ; the 

 tide, in its rise and fall, wiU cover and uncover equal arcs of this circle 

 in equal times. If this circle be divided like the dial of a clock, XII. 

 and VI. corresponding to the top and bottom of the vertical diameter, 

 and tidal hours be used, the rise or fall of the water may be easily cal- 

 culated. 



In calculating the Drift produced by the Tidal Stream, we are not 

 given the total drift in six tidal hours, which would correspond to the 

 Range of the Tide ; but we have instead the maximum velocity of the 

 Tidal Current at half-flood and half-ebb. 



The following construction will enable us easily to calculate the 

 Tidal Drift between two given hours : — 



Let a circle he described whose radius is double the maximum rate of 

 stream, and let this circle he divided into Tidal Sours ; from the two given 



E. I. A. TEOC— VOL. Vlll. 



E 



