1822.] Mr, Marratt on Sluice-Doors and Flood-Gates, 355 



cerithia. The shells which I have described by the generic 

 term potamides belong exclusively to the species ventricosus and 

 acutiis, figured in plate 341. In their external characters, they 

 are sufficiently distinguished ft-om cerithia, and they are so inti- 

 mately associated with freshwater shells that it is hardly possible 

 for them to have been of marine origin. An enumeration of the 

 fossils subsequently figured by Mr. Sowerby (plates 339, 340), 

 was intentionally omitted, as they were considered of doubtful 

 origin ; and, therefore, of no assistance in separating Xhe, forma- 

 tions. It is very important that those who collect fossils on the 

 Hampshire coast should describe the localities more carefully 

 than they have been in the habit of doing. P. rigidus, pi. 338, 

 is probably derived from some of the sandy beds which separate 

 the London clay from the loiver freshwater formation. A few 

 freshwater shells may be expected in such a marine deposit. 

 To avoid all ambiguity, would it not be better to expunge the 

 genus pot amides y and consider all the species as cerithia? Those 

 which are inhabitants of freshwater might be distinguished from 

 the others by some epithet, which would answer the purpose 

 better than the artifice of making a new genus without any new 

 generic characters. The fragments of the bulimus ellipticus 

 (Min. Con. t. 337, f. 2), were found in the highest bed of i\^e 

 upper freshwater rock of Headen Hill, 



Article IV. 



A New Method of hanging Sluice-Doors and Flood-Gates, 

 By W. Marratt, AM. 



(To the Editor of the Annals of Fhilosophy.) 



SIR, Liverpool. 



The usual method of hanging doors which are intended to 

 keep up or let out water from sluices, dams, &c. is either to 

 place them so that they may be opened like common doors, or 

 else to elevate and let them down by machinery ; in both cases, 

 the operation is often very tedious and troublesome. The fol- 

 lowing method, which, for any thing that I know, is new, and 

 has not yet been put in practice, is certainly in many cases pre- 

 ferable to the methods just alluded to. 



Let a rectangular door be fitted to the place for which it is 

 designed, and let it be hung, by placing across it strong gud- 

 geons, which must turn in holes made in the jambs, or in a 

 wooden frame placed for the purpose ; or they may play in cir- 

 cular holes made in the stone work. The proper situation for 

 the gudgeons on the door may be thus determined : Draw two 



2 a2 



