App.C] 



Dr. PiTTON on the Strata below the Chalk. 



375 



7. Railroad Sections. — From the uncertainty of the tide levels, and the inherent defects of the 

 methods, both trigonometric and barometrical, by which most of the heights above mentioned 

 were obtained, they cannot be absolutely depended on within a few feet. Those given in the 

 sections of the numerous railroads through England, having been ascertained by levelling, are 

 much more to be relied upon : they are, probably, as accurate as any that ever have been 

 obtained in undertakings not expressly directed to scientific purposes ; and are the more valuable 

 as the two other modes of measurement are most erroneous when the elevations are small. 

 In the course of a few years the entire surface of this country, wherever the introduction of 

 railroads is practicable, will thus be covered with a network of intersecting lines, along every 

 one of which the heights will have been determined to within a few inches of the truth. A 

 well-arranged collection of the levels will then form a very useful compilation to inquirers in 

 various departments of physical science ; and it is to be hoped that some person properly quali- 

 fied, who has leisure for such a purpose, may be induced to enter upon the undertaking*. 



The following Lists contain only the approximate height of a small number of points, alono- 

 the principal lines of railroad in that part of the South-East of England which is represented in 

 the map, PI. IX. The first two cut through the chalk range of the midland counties, in passing 

 from London ; — 1. North-westward, as far as Stony Stratford; — 2. By Reading, through the de- 

 file of the Thames, towards Abington ; and thence, westward, to the vicinity of Chippenham. 

 3. The South-eastern line, after passing the chalk near Godstone, descends the Valley of Kent 

 by Tonbridge and Ashford, to Folkstone and Dover. 4. and 5. The Brighton lines traverse the 

 chalk downs and the Wealden denudation, southwards, between London and the Sussex coast. 

 6. The Southampton railroad crosses the continuation of the anticlinal line of the Wealden to- 

 wards the Vale of Wardour, and connects the basins of London and the Isle of Wight. 



I.— HEIGHTS ON PART OF THE LONDON AND BIRMINGHAM RAILWAYf. 



Stations. 



The Datum in the Section of this Railroad, 

 is the same with that of Bradshaw's map 

 of canals ; viz. Low-water mark at the Old 

 Docks, Liverpool, (6 Ft. 10 In. under the 

 Dock sill.) 



Datum below the Trinity high water mark, at 

 the London Docks: (Bradshaw)...21 feet. 



[The datum at Liverpool, therefore, is be- 

 neath low-water mark at Sheerness, 

 (21 — 16-6699 =) 1-330 feet] 



Height of railway above the datum at the 

 Regent's Canal near Camden Town 



Height 



above the 



Datum. 



Feet. 



118-75 



Stations. 



Highest point of the ground over the Tunnel 

 near Primrose Hill, about 0-1 mile north- 

 west of letter P 



Road from London to Edgware, at Kilburn 



South-east bank of the Brent, about a mile 

 east oi Apperton 



Road from Harrow to Harrow Weald, about 



\ mile beyond Greenhill 



Stanmore to Pinner, near Dove- 



house farm 



Highest point of ground over the Tunnel at 



Oxhey-lane: north of the e in lane 



Height 



above the 



Datum. 



Feet. 



189-50 

 130 



87-6 



190 



218-08 



287 



* In the present unfinished state of the works, the attempt would be premature ; but when the railroads 

 have been completed, the several lines will furnish admirable facilities for accurate levelling along them, and for 

 connecting the heights in the interior of England, with fixed and durable standards at well-chosen places on 

 the coast, the height of which above the sea, may be afterwards ascertained by long-continued observation. 

 From the first of these operations, we should obtain the re/a^iye height of a number of points, any one of which 

 might be taken as a standard, and all of which are, probabl)', themselves in motion, and may be expected to 

 differ in elevation after the lapse of years. On this account, the multiplication of standard marks, both inland 

 and on the shore, is very desirable ; while the latter should be compared, from time to time, with the supposed 

 invariable mean level of the sea. The height of the tides themselves, however, is in some degree affected by the 

 features of the coast, which it is well known to geologists, are in a state of constant and even rapid alteration. 



f Where roads, canals, and streams are mentioned in these lists, the points referred to are those of their 

 intersection by the railway. The names of the places are in the Ordnance Maps ; and the precise points are 

 indicated by reference to adjacent words, letters, or objects on the map. 



VOL IV. — SECOND SERIES. 



3c 



