304 On the Fall of Rivers. 



municates the following observations on the fall of rivers in 

 general, and on that of the Jordan, in Palestine, of the 

 Thames, Tweed, Clyde, and Dee, in Britain, and the Shan- 

 non, in Ireland. 



The data regarding the British rivers, Mr Petermann in- 

 forms us, " are a part of the results of laborious researches 

 into the physical and statistic geography of the British 

 Isles, which he hopes ere long to lay before the public in a 

 series of maps. 



1. The Fall of Rivers. — This interesting branch of physical 

 geography presents, in its comparative results, such strik- 

 ing anomalies as perhaps have never before been anticipated. 

 There are rivers in this country that are of the same aggre- 

 gate size and descent, and the one forms in its course a 

 series of considerable waterfalls, the heights of some of 

 which approach nearly to 100 feet ; whilst the other, equi- 

 valent, as before mentioned, in size and fall, presents not 

 even a single waterfall or cataract, nay, not even one decided 

 rapid. 



To ascertain the rate of fall, there are two points, the ac- 

 curate knowledge of which is necessary,- — 1st, The length of 

 the river in question ; 2d, Its elevation. 



The length of the river, that is to say, the development of 

 its course, is greatly influenced by the extent of its windings. 

 These windings, in their true extent, can only be delineated 

 with sufficient accuracy in maps of a very large scale, such 

 as is adapted for national maps, as the windings which rivers 

 generally exhibit disappear in reduced maps to such a degree 

 that a great difference is produced in the calculations. 



For the purpose of forming a judgment of the rate of the 

 decrease of a river's course by constructing different maps in 

 decreasing scales, I made the following inquiry regarding a 

 river which is not uncommonly meandering in its course. I 

 here allude to a portion of the Severn, from its source to 

 Shrewsbury, and I found the following numerical results : — 



