757 



If doubly refracting media are used as plates, two sets of bands 

 are seen superimposed, which are easily shown to be due to the or- 

 dinary and extraordinary rays respectively. But for some of these 

 crystals the data are as yet insufficient to give more than a general 

 accordance with theory. 



The method might be applied practically for determining the in- 

 dices of many substances to which the ordinary method is inapplica- 

 ble from the impossibility of forming them into prisms : n will also 

 exhibit palpably the most insensible degree of double refraction, and 

 may thus become useful to the mineralogist. 



There is a close analogy between these phenomena and those 

 observed by Baron von Wrede, and by Sir D. Brewster and Mr. 

 Fox Talbot, of which Mr. Airy has given a theory. A similar 

 theory is necessary for explaining some of the more minute details 

 of the present phenomena ; and on this subject some extensive re- 

 searches have been pursued by Mr. Stokes of Pembroke College, 

 Cambridge, which will soon appear. 



" On the Meteorology of the Lake District of Cumberland and 

 Westmoreland." By John Fletcher Miller, Esq. Communicated by 

 Lieut.. Col. Sabine, R.A., For. Sec. R.S. 



The author has devoted nearly four years to the investigation of 

 the quantities of rain falling in the lake districts of Cumberland and 

 Westmoreland ; and he commenced, two years ago, a set of experi- 

 ments specially directed to ascertain the amount of rain deposited at 

 great elevations above the sea, such as the summits of our highest 

 English mountains. As the investigation proceeded, some remarka- 

 ble results were obtained, which coming to the knowledge of the 

 Royal Society early in last year (1847), the Council contributed a 

 sum of money from the Donation Fund towards the current expenses 

 attending this inquiry, of which the results are given in the present 

 communication, comprising extensive tables of observations relative 

 to the quantity of rain in different situa^tions within the above period 

 of time. 



May 25, 1848. 



The xMARQUIS OF NORTHAMPTON, President, in the Chair. 



" On the structure of the Jaws and Teeth of the Iguanodon." 

 By Gideon Algernon Mantell, Esq., LL.D., F.R.S., Vice-President 

 of the Geological Society, &c. 



The recent discovery of the right dentary bone of the lower jaw 

 of an adult Iguanodon with teeth, having enabled the author, with 

 the aid afforded by other specimens, to determine the structure of the 

 maxillary organs of that gigantic herbivorous reptile, the result of 

 his investigations are embodied in the present communication. 



The first memoir of the author on the teeth of the Iguanodon was 

 published in the Philosophical Transactions for 1825 ; but owing 



