202 On Constructing Geological Models. 



the scale of the model. The scale which I have adopted is as 

 nearly as possible six inches to the mile; so that if the 

 Ordnance map be used it will only be necessary to cover it 

 with a number of squares, and a sheet of paper with the same 

 number six times larger, and then copy the map from one to 

 the other ; if, on the other hand, the Tithe Commutation maps 

 of six chains to the inch have been used, if the tracing be 

 covered with inch squares, and the blank paper with squares 

 of *45 of an inch (which can be very readily marked off upon 

 it by using a common scale of two chains to the inch), it will 

 then be found that whatever is copied on the blank paper in 

 the proportion of these squares, will have very nearly the 

 required dimensions of six inches to the mile. This plan being 

 then pasted on a board the size of the intended model, a 

 number of strips of tin must be cut, one-eighth, one-fourth, 

 three-eighth inches broad, and so on, allowing an eighth of 

 an inch for every twenty-five feet in altitude. This proportion 

 is indeed considerably greater than the truth; but in such 

 models and in all sections it is found quite necessary to 

 exaggerate the altitudes proportionally to the horizontal 

 measurements, since from the position of our eyes relatively to 

 the model, we cannot see the miniature hills and valleys as we 

 do in nature, and require this artifice to make them look 

 correct. However, as the proportions are always observed, no 

 real inaccuracy results. 



We must now fix these strips of tin on edge upon the board 

 by means of small tacks, the eighth-inch strip upon the 

 twenty-five feet contour line, the quarter-inch upon the fifty 

 feet line, and so on till the board is covered with them, rising 

 behind each other like steps, and following in miniature the 

 same curves which the lines did that we traced on the hill- 

 sides : all that now remains is to fill in between these pieces of 

 tin with any composition, such as clay or plaster ; and if it be 

 desired afterwards, a cast of the whole can be taken in the 

 usual manner, and be coloured according to the various 

 geological strata, and their dip represented by slight Hues 

 drawn upon the surface. 



I can assure those who may undertake such a model that 

 the result is highly satisfactory. By the expenditure of a little 

 time and trouble it is thus possible to obtain an exact fac- 

 simile of the surface of the earth, and to show the effects of 

 upheaval and denudation, and which, wherever, as in my own 

 neighbourhood, they are distinctly marked, cannot fail to be 

 both highly instructive and interesting, besides that as the 

 survey progresses the notes which may be made of the dip and 

 condition of the strata may lead to a more intimate knowledge 

 of them, and thus to ascertaining some important facts 

 unobserved in a more cursory survey. 



