141 



21. — The Heetfoedshiee OEcxAJifCE Bench IEaeks, feom the 



'AbSTEACTS of LEYELLIIfa' OF IHE OEDXAIfCE SuEVET. 



Commimicated by John Hopkixson, Hon. Sec. 



[Read 9th November, 1876.] 



The work from which the following levels are taken* gives the 

 height, above the Datum Level for Great Britain — the level of mean 

 tide at Liverpool — of every bench mark in sixty-three separate lines 

 of levelling, with branches to trigonometrical stations situated near 

 the different lines. "The levels established on these lines, and 

 marked /f\ upon permanent objects on the ground, furnish definite 

 points all over the kingdom from which the lines of levels for the 

 contours and the detail plans of eveiy part of the country are carried, 

 and thus all the levels on the Ordnance plans refer to one common 

 datum level." f 



Two of these lines of levelling run through Hertfordshire. One 

 is that from Birmingham to London. It is in an almost direct 

 line fi'om north-west to south-east, following the coach road 

 through the towns of Coventry, Dunchurch, Towcester, Dunstable, 

 St. Albans, and Barnet. It was commenced on the 19th of 

 January, 1848, and completed on the 7th of April in the same 

 year. The other is that from London to Dunstable. This takes 

 a much more circuitous route, being first carried in a northerly 

 direction along the road through the towns of Tottenham, Ches- 

 hunt, and Hoddesdon, to Ware, thence turning west through 

 Hertford, north-west through Stevenage to Hitchin, south-west 

 to Luton, and again north-west to Dunstable, where it joins the 

 former line. It was commenced on the 11th of July, 1848, and 

 completed on the 26th of July, 1851. 



The height above the datum level, as already defined, is given 

 in feet, and earned to three places of decimals, or to the thousandth 

 part of a foot. 



There are two kinds of bench marks. The "Mark," which 

 consists of a vertical broad arrow cut into some permanent object, 

 and a horizontal line above it, the centre of which indicates the 

 exact level; and the "Bolt," a first-class "Mark," which is dis- 

 tinguished by a piece of copper bolt driven into the object, which 

 in this case is usually the stone wall of a church tower, a public 

 building such as a town-hall, or a bridge. 



Besides these levels, others are given by the Ordnance Survey in 

 the " Parish Maps " on the scale of ^Jto, or 25-344 inches to the 

 mile, but the maps of only a few parishes in the county are as yet 

 published. The parish of "Watford has recently been completed. 



* ' Abstracts of the Principal T-ines of Spirit Levelling in England and 

 Wales.' By Colonel Sir Henry James, R.E., F.R.S , etc.. Director of the 

 Ordnance Survey. 1861. 



t I.e. p. v. 



