On the Diallage Rock of Shetland. 103 



centre of flame (A) to (a), so that the greatest quantity of oH is con< 

 tained in a form that gives the least possible shadow. 



(C) The metallic Reflector fixed on the Glass Chimney close to the Flame, 

 by metallic springs, and reflecting^ some of the brightest light upon 

 the table. 



(D) The Frosted Glass Light Distributor. 



The use of the Reflector with the Distributor gives the Sinambra 

 Lamp a great advantage in point of economy, as a very considerable 

 increase of light is downwards obtained without any additional 

 consumption of oil. 



(E) The Pipes to conduct the oil from the Reservoir to the Burner. 

 <F) The Glass Chimney. 



(G) The Button, to be unscrewed when the oil is introduced. 

 Fig. 3, represents the Lamp in its complete state. 



Art. X. On the Diallage Rock of Shetland. Bj/ J. Mac 

 CuUoch, M.D. F.R.S., ^c, in a Letter to the Editor, 



Dear Sir, Shetland, August 20, 1820. 



As I am not aware that the relations of Diallage 

 Rock have been described by any author, a brief account of its 

 character, as it occurs in this country, will probably not be un- 

 acceptable to the readers of your Journal. I presume that we 

 are indebted to Dr. Hibbert for having first pointed out its 

 existence in these islands. I am, &c. 



To W, T. Brands, Esq. J. Mac Culloch. 



Royal Institution. 



Diallage Rock forms a considerable part of the island of 

 Unst, and a small proportion of FeUar. It also constitute^ 

 Houna, and the whole of Balta ; an island rendered remarkable 

 of late in the annals of astronomy, from its having been the 

 northernmost point of that arc of the meridian measured by thiQ 

 late Generals Roy and Mudge, under the direction of the Board 

 of Ordnance. It also occurs, but in very small quantity, at 

 Fedaland point, the northernmost promontory of the main land 

 of Shetland, and at Hills wick Ness. 



In this district it cannot be said to confer any particular cha- 

 raoter on the general form of the land ; which, whatever rock is 



