94 EIGHTH REPORT 1838. 



those which he has delineated, and in situations where it is hardly con- 

 ceivable that he could have overlooked them ; and that in other places 

 those which he has represented now exhibit a difference in magnitude 

 which cannot well be explained by any supposition of accidental haste 

 or inaccuracy. The charts of Lohrmann unfortunately do not contain 

 those portions of the lunar surface in which Mr. Webb conceives these 

 alterations to have taken place ; but he had the pleasure of finding 

 several of his observations confirmed by the beautiful Mappa Seleno- 

 graphica of Messrs. Beer and Miidler ; and he expresses a hope that a 

 more extended and accurate investigation may, in the course of a few 

 years, not only bring to light the progress of many interesting changes, 

 but may even enable us to form some inferences as to the nature and 

 mode of action of that power which has produced such extensive and 

 multiplied revolutions upon the lunar surface. 



On the Construction of Geological Models. By Thomas Sopwith, 



F.G.S. 



Next to the actual inspection of any object, a model is the most per- 

 fect means of conveying a clear idea of the general appearance and 

 construction of the object which it represents ; and in some cases a 

 model exhibits and explains details which either cannot be grasped at 

 once by the eye in the real object, or which are hidden from actual in- 

 spection, as in the case of the geological structure of the earth, or the 

 interior of mines. 



Some difficulty, however, has been found in conveying to ordinary 

 workmen such a knowledge of geological or mining details, as shall 

 enable them to execute a work of so much intricacy as these subjects 

 usually present. The following method was pursued in the construc- 

 tion of a model of Dean Forest, and it is equally applicable to any geo- 

 logical model of a district. 



Being in possession of an accurate plan and sections of the district, 

 derived from surveys which he had made for the Commissioners of 

 Woods and Forests in 1834, Mr. Sopwith divided the tract of country, 

 comprehending about thirty-six square miles, by two series of parallel 

 lines intersecting each other at the distance of a mile from each other. 

 A vertical section was then prepared, corresponding with each of these 

 lines. These several sections were drawn upon tiiin pieces of wood, 

 which were united together by being what workmen term half lapped, 

 forming a skeleton model of vertical sections. After having been united, 

 the sections were taken separately, and cut into portions corresponding 

 with the contour of the several layers of strata to be represented, the 

 corresponding points of intersection having been previously marked 

 with figures. These respective portions are again united, to form the 

 exterior boundary or vertical edge of a square mile of rocks. The in- 

 terior of each of these squares is filled with wood, and carved so as to 



