Again, taking the London district as an example, Prestwich remarks that, as the country rises on both sides of the Thames to the edge of the chalk escarpments, and as the outcrop of the Lower Tertiary strata is intermediate between these escarpments and the Thames, it follows that the outcrop of these lower beds must, in all cases, be on a higher level than the Thames itself, where it flows through the centre of the Tertiary district. Its altitude is, of course, very variable, as shown in the following list of its approximate height above Trinity high water-mark at London. These heights are taken where the Tertiaries are at their lowest level in the several localities mentioned. --------------------------------+------------------------- South of London. | North of London. --------------------------------+------------------------- Croydon about 130 feet. | Thetford about 200 feet. Leatherhead " 90 " | Watford " 170 " Guildford " 96 " | Slough " 60 " Old Basing " 250 " | Reading " 120 " Near Hungerford " 360 " | Newbury " 236 " --------------------------------+------------------------- Eastward of London these strata crop out at a gradually decreasing level. In consequence, therefore, of the outcrop of the water-bearing strata being thus much above the surface of the central Tertiary district bordering the Thames, the water in these strata beneath London tended originally to rise above that surface. As, however, these beds crop out on a level with the Thames immediately east of the city between Deptford, Blackwall, and Bow, the water, having this natural issue so near, could never have risen in London much above the level of the river. RAINFALL IN THE DISTRICT WHERE THE WATER-BEARING STRATA CROP OUT. When inquiring into the probable relative value of any water-bearing strata, it is necessary to compare the rainfall in their respective districts. Rain is of all meteorological phenomena the most capricious, both as regards its frequency and the amount which falls in a given time. In some places it rarely or never falls, whilst in others it rains almost every day; and there does not yet exist any theory from which a probable estimate of the rainfall in a given district can be deduced independently of direct observation. But although dealing with one of the most capricious of the elements, we nevertheless find a workable average in the quantity of rain to be expected in any particular place, if careful and continued observations are made with the rain-gauge. G. J. Symons, the meteorologist, to whose continued investigations we are indebted for our most reliable data upon the subject of rainfall, gives the following practical instructions for using a rain-gauge;--