2i> 



every facility possible is afforded for the emptying of the vessels of 

 these processes. He hence infers that the office of these processes 

 is to allow of the displacement of fluid when the lens is carried for- 

 wards ; and when the choroid muscle ceases to act, by the re-disteu- 

 sion of their capillaries, to carry the lens back into its place. The 

 author then enters into a mathematical examination of the data fur- 

 nished by these facts, to show how exactly they fulfill the conditions 

 necessary for adapting the eye, viewed as an optical instrument, to 

 distinct vision at different distances. The author considers that 

 there is no analogy either in structure or function between those 

 fibres in mammals occupying a situation similar to that occupied by 

 a true muscle in Birds (the ciliary muscle), but that the true ana- 

 logue of the ciliary muscle in birds is the choroid m.uscle in mam- 

 mals, the chief difference between them being in situation. In the 

 Pigeon, he finds that the ciliary muscle is inserted into the choroid 

 coat, along which it can be traced as far back as about ith of an 

 inch behind the iris, so that its action v/ould be to draw the choroid 

 tense upon the vitreous humour, and thus to compress it similarly to 

 the choroid muscle in the mamm.al. Besides, in the Bird he finds 

 no muscular fibres at the posterior part of the choroid. The ciliary 

 muscle is more distinct, and appears to be stronger than the choroid, 

 but this the author attributes to the fibres of the one being much 

 more collected, and therefore limited to a much smaller space than 

 those of the other. 



February 6, 1851. 

 GEORGE RENNIE, Esq., V.P., in the Chair. 



A paper was read, entitled " On the Supply of Water from the 

 Chalk Stratum in the neighbourhood of London." By John 

 Dickinson, Esq., F.R.S. Received January 6, 1851. 



The object of this paper is to explain and illustrate the supply of 

 subterranean water which is ahvays found at certain depths in the 

 chalk strata; the circumstances that influence its natural outflow 

 by springs and rivers ; the practicability of draining off that Vvater 

 by an artificial mode of exhaustion ; and the changes that would 

 be produced by carrying such an operation into effect on a large 

 scale. 



It is stated, that numerous perennial stream.s issue from the ele- 

 vated ridges of the chalk strata, those in Kent and Surrey flowing 

 from south to north, and those in Buckinghamshire, Herts and 

 Essex, flowing from north to south ; and that in each case the dip of 

 the strata corresponds with the fall of the country and the direction 

 of the streams. These rivers are considered to be the natural out- 

 flow of the rain-water im.bibed by the chalk, the accumulation of 

 which, as explained by Dr. Buckland, is in a subterranean reservoir ; 

 and according to the periodical filling and exhaustion of this, the 

 springs and streams alternately decrease and are augmented. The 

 circumstances regulating this change, which have been observed and 



