[From the Philosophical Magazine for December, 1870.] 



XLIII. On Hills and Dales. 



To the Editors of the Philosophical Magazine and Journal. 



GENTLEMEN, 



I FIND that in the greater part of the substance of the following 

 paper I have been anticipated by Professor Cayley, in a memoir " On Contour 

 and Slope Lines," published in the Philosophical Magazine in 1859 (S. 4. Vol. 

 xvin. p. 264). An exact knowledge of the first elements of physical geography, 

 however, is so important, and loose notions on the subject are so prevalent, 

 that I have no hesitation in sending you what you, I hope, will have no 

 scruple in rejecting if you think it superfluous after what has been done by 

 Professor Cayley. 



I am, Gentlemen, 



Your obedient Servant, 



J. CLERK MAXWELL. 



GLENLAIR, DALBEATTIE, 



October 12, 1870. 



1. ON CONTOUR-LINES AND MEASUREMENT OF HEIGHTS. 



The results of the survey of the surface of a country are most conveniently 

 exhibited by means of a map on which are traced contour-lines, each contour-line 

 representing the intersection of a level surface with the surface of the earth, 

 and being distinguished by a numeral which indicates the level surface to 

 which it belongs. 



When the extent of country surveyed is small, the contour-lines are defined 

 VOL. II. 30 



