Notes and Comments. 195 



■of Gilbert White. Not only has the money been forthcoming 

 to buy the wood which for 17 years has been maintained as the 

 Brent Valley Bird Sanctuary, but also some additional 

 land which the owners have been so good as to sell at a special 

 price,, seeing that the whole 26 acres will remain an open 

 space for ever. The fund which has now been started for the 

 upkeep and endowment of the new Gilbert White Memorial 

 is progressing., and the latest contribution is a gift that will 

 Turing in £26 per annum. 



RARE BIRDS. 



Although quite close to London, and indeed coming into 

 its postal district, many birds with which it might not be 

 expected to meet, are to be seen in the Sanctuary. Among 

 those which have nested (about 50 in number) are the Sparrow 

 Hawk, Greater Spotted Woodpecker, WVyneck, Turtle -Dove, 

 Hawfinch, Goldfinch and Wild Duck, while visitors include 

 the Kingfisher, Magpie, Nightjar, Golden Crested Wren, 

 Snipe, and Woodcock. It is in the wood that the experiments 

 were carried out which resulted in the making of Selborne 

 Nesting Boxes which have been sent all over this country, 

 and to many parts of the world, particularly with a view 

 to increasing the number of insect -eating birds which are 

 beneficial to man. The Secretary to the Committee is Mrs. 

 Wilfred Mark Webb, of the Hermitage, Hanwell, London, W.7. 



THE LEVEL OF THE SEA. 



We learn from the Westminster Gazette that ' when the 

 primary levelling of the country was commenced in 1841 the 

 'datum line was 100 ft. below a mark on St. John's Church, 

 'Old Haymarket, Liverpool ; but shortly afterwards it was 

 changed to mean sea level of the Mersey at Liverpool, which 

 was assumed to be 4" 67 ft. above the zero of the tide guage at 

 Georges Ferry basin. This is the datum from which the 

 altitudes on the Ordnance Survey maps are computed, but it 

 is now known that the Mersey datum is eight inches below 

 true mean sea level, so that all Ordnance Survey altitudes 

 are too high by that amount. The altitude mark outside 

 Greenwich Observatory is 154 ft., above mean water at 

 Greenwich and 155' 7 ft. above that at Liverpool, the difference 

 being ascertained by a line of levels run through Blackheath, 

 Dunstable, Arbury Hill, Bardon Hill, Axe Edge, and Whittle 

 Hill, Lancashire.' 



THE NEW ' BENCH MARKS.' 



All over the country altitudes calculated from the Liverpool 

 datum are shown on 'bench marks ' placed on convenient 

 walls, bridges, milestones, and even private houses. But 

 these are being superseded by 8 ft. long granite pillars sunk 

 in the ground and embedded in concrete, with the altitudes 

 engraved on a brass plate on the top. These altitudes are 



1921 J4ine 1 



