Such data as the above may be used in approximately estimating the probable available rainfall of a district; but a much more accurate and satisfactory method is to measure the actual discharge of the streams, and the quantity lost by evaporation, at the same time that the rain-gauge observations are made, and so to find the actual proportion of available to total rainfall.
The following Table gives the mean annual rainfall in various parts of the world;—
| Table of Rainfall. Collected by G. J. Symons. |
| Country and Station. | | Period
of
Observations. | Latitude. | Mean
Annual
Fall. |
| | | | | |
| EUROPE. | | years | ° ′ | ins. |
| Austria—Cracow | | 5 | 50 4N | 33·1 |
| Prague | | 47 | 50 5 | 15·1 |
| Vienna | | 10 | 48 12 | 19·6 |
| Belgium—Brussels | | 20 | 50 51 | 28·6 |
| Ghent | | 13 | 51 4 | 30·6 |
| Louvain | | 12 | 50 33 | 28·6 |
| Denmark—Copenhagen | | 12 | 55 41 | 22·3 |
| France—Bayonne | | 10 | 43 29 | 56·2 |
| Bordeaux | | 32 | 44 50 | 32·4 |
| Brest | | 30 | 48 23 | 38·8 |
| Dijon | | 20 | 47 14 | 31·1 |
| France—Lyons | | .. | 45 46 | 37·0 |
| Marseilles | | 60 | 43 17 | 19·0 |
| Montpelier | | 51 | 43 36 | 30·3 |
| Nice | | 20 | 43 43 | 55·2 |
| Paris | | 44 | 48 50 | 22·9 |
| Pau | | 12 | 43 19 | 37·1 |
| Rouen | | 10 | 49 27 | 33·7 |
| Toulon | | .. | 43 4 | 19·7 |
| Toulouse | | 52 | 43 36 | 24·9 |
| Great britain— | | | | |
| England, London | | 40 | 51 31 | 24·0 |
| „ Manchester | | 40 | 53 29 | 36·0 |
| „ Exeter | | 40 | 50 44 | 33·0 |
| „ Lincoln | | 40 | 53 15 | 20·0 |
| Wales, Cardiff | | 40 | 51 28 | 43·0 |
| „ Llandudno | | 40 | 53 19 | 30·0 |
| Scotland, Edinburgh | | 40 | 55 57 | 24·0 |
| „ Glasgow | | 40 | 55 52 | 39·0 |
| „ Aberdeen | | 40 | 57 8 | 31·0 |
| Ireland, Cork | | 40 | 51 54 | 40·0 |
| „ Dublin | | 40 | 53 23 | 30·0 |
| „ Galway | | 40 | 53 15 | 50·0 |
| Holland—Rotterdam | | .. | 51 55 | 22·0 |
| Iceland—Reikiavik | | 5 | 64 8 | 28·0 |
| Ionian Isles—Corfu | | 22 | 39 37 | 42·4 |
| Italy—Florence | | 8 | 43 46 | 35·9 |
| Milan | | 68 | 45 29 | 38·0 |
| Naples | | 8 | 40 52 | 39·3 |
| Rome | | 40 | 41 53 | 30·9 |
| Turin | | 4 | 45 5 | 38·6 |
| Venice | | 19 | 45 25 | 34·1 |
| Malta | | .. | 35 54 | 15·0 |
| Norway—Bergen | | 10 | 60 24 | 84·8 |
| Christiania | | .. | 59 54 | 26·7 |
| Portugal—Coimbra (in Vale of Mondego) | | 2 | 40 13 | 224·0? |
| Lisbon | | 20 | 38 42 | 23·0 |
| Prussia—Berlin | | 6 | 52 30 | 23·6 |
| Cologne | | 10 | 50 55 | 24·0 |
| Hanover | | 3 | 52 24 | 22·4 |
| Potsdam | | 10 | 52 24 | 20·3 |
| Russia—St. Petersburg | | 14 | 59 56 | 16·2 |
| Archangel | | 1 | 64 32 | 14·5 |
| Astrakhan | | 4 | 46 24 | 6·1 |
| Finland, Uleaborg | | .. | 65 0 | 13·5 |
| Sicily—Palermo | | 24 | 38 8 | 22·8 |
| Spain—Madrid | | .. | 40 24 | 9·0 |
| Oviedo | | 1 | 43 22 | 111·1 |
| Sweden—Stockholm | | 8 | 59 20 | 19·7 |
| Switzerland—Geneva | | 72 | 46 12 | 31·8 |
| Great St. Bernard | | 43 | 45 50 | 58·5 |
| Lausanne | | 8 | 46 30 | 38·5 |
| | | | |
| ASIA. | | | | |
| China—Canton | | 14 | 23 6 | 69·3 |
| Macao | | .. | 22 24 | 68·3 |
| Pekin | | 7 | 39 54 | 26·9 |
| India— | | | | |
| Ceylon, Colombo | | .. | 6 56 | 91·7 |
| „ Kandy | | .. | 7 18 | 84·0 |
| „ Adam’s Peak | | .. | 6 50 | 100·0 |
| Bombay | | 33 | 18 56 | 84·7 |
| Calcutta | | 20 | 22 35 | 66·9 |
| Cherrapongee | | .. | 25 16 | 610·3? |
| Darjeeling | | .. | 27 3 | 127·3 |
| Madras | | 22 | 13 4 | 44·6 |
| Mahabuleshwur | | 15 | 17 56 | 254·0 |
| Malabar, Tellicherry | | .. | 11 44 | 116·0 |
| Palamcotta | | 5 | 8 30 | 21·1 |
| Patna | | .. | 25 40 | 36·7 |
| Poonah | | 4 | 18 30 | 23·4 |
| Malay—Pulo Penang | | .. | 5 25 | 100·5 |
| Singapore | | .. | 1 17 | 190·0 |
| Persia—Lencoran | | 3 | 38 44 | 42·8 |
| Ooroomiah | | 1 | 37 28 | 21·5 |
| Russia—Barnaoul | | 15 | 53 20 | 11·8 |
| Nertchinsk | | 12 | 51 18 | 17·5 |
| Okhotsk | | 2 | 59 13 | 35·2 |
| Tiflis | | 6 | 41 42 | 19·3 |
| Tobolsk | | 2 | 58 12 | 23·0 |
| Turkey-Palestine, Jerusalem | { | 14
3 | 31 47
31 47 | 65·0?
16·3 |
| Smyrna | | .. | 38 26 | 27·6 |
| | | | | |
| AFRICA. | | | | |
| Abyssinia—Gondar | | .. | 12 36 | 37·3 |
| Algeria—Algiers | | 10 | 36 47 | 37·0 |
| Constantina | | .. | 36 24 | 30·8 |
| Mostaganem | | 1 | 35 50 | 22·0 |
| Oran | | 2 | 35 50 | 22·1 |
| Ascension | | 2 | 8 8S | 11·5 |
| Cape Colony—Cape Town | | 20 | 33 52 | 24·3 |
| Guinea—Christiansborg | | .. | 5 30N | 19·2 |
| Madeira | | 4 | 33 30 | 30·9 |
| Mauritius—Port Louis | | .. | 20 3S | 35·2 |
| Natal—Maritzburgh | | .. | 29 36 | 27·6 |
| St.Helena | | 3 | 15 55N | 18·8 |
| Sierra Leone | | .. | 8 30 | 86·0 |
| Teneriffe | | 2 | 28 28 | 22·3 |
| | | | | |
| NORTH AMERICA. | | | | |
| British Columbia—New Westminster | | 3 | 49 12 | 54·1 |
| Canada—Montreal, St. Martin’s | | 2 | 45 31 | 47·3 |
| Toronto | | 16 | 43 39 | 31·4 |
| Honduras—Belize | | 1 | 17 29 | 153·0 |
| Mexico—Vera Cruz | | .. | 19 12 | 66·1 |
| Russian America—Sitka | | 7 | 57 3 | 89·9 |
| United States—Arkansas, Fort Smith | | 15 | 35 23 | 42·1 |
| California, San Francisco | | 9 | 37 48 | 23·4 |
| Nebraska, Fort Kearny | | 6 | 40 38 | 28·8 |
| New Mexico, Socorro | | 2 | 34 10 | 7·9 |
| New York, West Point | | 12 | 41 23 | 46·5 |
| Ohio, Cincinnati | | 20 | 39 6 | 46·9 |
| Pennsylvania, Philadelphia | | 19 | 39 57 | 43·6 |
| South Carolina, Charlestown | | 15 | 32 46 | 48·3 |
| Texas, Matamoras | | 6 | 25 54 | 35·2 |
| West Indies—Antigua | | .. | 17 3 | 39·5 |
| Barbadoes | | 10 | 13 12 | 75·0 |
| „ St. Philip | | 20 | 13 13 | 56·1 |
| Cuba, Havannah | | 2 | 23 9 | 50·2 |
| Matanzas | | 1 | 23 2 | 55·3 |
| Grenada | | .. | 12 8 | 126·0 |
| Guadaloupe, Basseterre | | .. | 16 5 | 126·9 |
| „ Matonba | | .. | 16 5 | 285·8 |
| Jamaica, Caraib | | .. | 18 3 | 97·0 |
| „ Kingstown | | .. | 17 58 | 83·0 |
| St. Domingo, Cape Haitien | | .. | 19 43 | 127·9 |
| „ Tivoli | | .. | 19 0 | 106·7 |
| Trinidad | | .. | 10 40 | 62·9 |
| Virgin Isles, St. Thomas’ | | .. | 18 17 | 60·6 |
| „ Tortola | | .. | 18 27 | 65·1 |
| | | | | |
| SOUTH AMERICA. | | | | |
| Brazil—Rio Janeiro | | .. | 22 54S | 58·7 |
| S. Luis de Maranhao | | .. | 3 0 | 276·0 |
| Guyana—Cayenne | | 6 | 4 56 | 138·3 |
| Demerara, George Town | | 5 | 6 50 | 87·9 |
| Paramaribo | | .. | 6 0 | 229·2 |
| New Granada—La Baja | | 6 | 7 22 | 54·1 |
| Marmato | | 15 | 5 29 | 90·0 |
| Santa Fé de Bogota | | 6 | 4 36 | 43·8 |
| Venezuela—Cumana | | .. | 10 27 | 7·5 |
| Curaçoa | | .. | 12 15N | 26·6 |
| | | | | |
| AUSTRALIA. | | | | |
| New South Wales—Bathurst | | 3 | 33 24S | 22·7 |
| Deniliquin | | 2 | 35 32 | 13·8 |
| Newcastle | | 3 | 32 57 | 55·3 |
| Port Macquarie | | 12 | 31 29 | 70·8 |
| Sydney | | 6 | 33 52 | 46·2 |
| New Zealand—Auckland | | 2 | 36 50 | 31·2 |
| Christchurch | | 3 | 43 45 | 31·7 |
| Nelson | | 2 | 41 18 | 38·4 |
| Taranaki | | 2 | 39 3 | 52·7 |
| Wellington | | 2 | 41 17 | 37·8 |
| South Australia—Adelaide | | 6 | 34 55 | 19·2 |
| Tasmania—Hobart Town | | 12 | 42 54 | 20·3 |
| Victoria—Melbourne | | 6 | 37 49 | 30·9 |
| Port Phillip | | 11 | 38 30 | 29·2 |
| West Australia—Albany | | .. | 35 0 | 32·1 |
| York | | 1 | 31 55 | 25·4 |
| | | | | |
| POLYNESIA. | | | | |
| Society Islands—Tahiti, Papiete | | 5 | 17 32 | 45·7 |
Disturbances of the Strata.
The last question to be considered relates to the disturbances which may have affected the strata; for whatever may be the absorbent power of the strata, the yield of water will be more or less diminished whenever the channels of communication have suffered break or fracture.
If the strata remained continuous and unbroken, we should merely have to ascertain the dimensions and lithological character of the strata in order to determine their water value. But if the strata is broken, the interference with the subterranean transmission of water will be proportionate to the extent of the disturbance.
Although the Tertiary formations around London have probably suffered less from the action of disturbing forces than the strata of any other district of the same extent in England, yet they nevertheless now exhibit considerable alterations from their original position.
The principal change has been that which, by elevation of the sides or depression of the centre of the district, gave the Tertiary deposits their present trough-shaped form, assuming it not to be the result of original deposition. If no further change had taken place we might have expected to find an uninterrupted communication in the Lower Tertiary strata from their northern outcrop at Hertford to their southern outcrop at Croydon, as well as from Newbury on the west to the sea on the east; and the entire length of 260 miles of outcrop would have contributed to the general supply of water at the centre.
But this is far from being the case; several disturbing causes have deranged the regularity of original structure. The principal one has caused a low axis of elevation, or rather a line of flexure running east and west, following nearly the course of the Thames from the Nore to Deptford, and apparently continued thence beyond Windsor. It brings up the chalk at Cliff, Purfleet, Woolwich, and Loampit Hill to varied but moderate elevations above the river level. Between Lewisham and Deptford the chalk disappears below the Tertiary series, and does not come to the surface till we reach the neighbourhood of Windsor and Maidenhead.
There is also, probably, another line of disturbance running between some points north and south and intersecting the first line at Deptford. It passes apparently near Beckenham and Lewisham, and then, crossing the Thames near Deptford, continues up a part, if not along the whole length of the valley of the Lea towards Hoddesdon. This disturbance appears in some places to have resulted in a fracture or a fault in the strata, placing the beds on the east of it on a higher level than those on the west; and at other places merely to have produced a curvature in the strata. Prestwich states that he was unable to give its exact course, but its effect, at all events upon the water supply of London, is important, as, in conjunction with the first or Thames valley disturbance, it cuts off the supplies from the whole of Kent, and interferes most materially with the supply from Essex; for in its course up the valley of the Lea it either brings up the Lower Tertiary strata to the surface, as at Stratford and Bow, or else, as farther up the valley, it raises them to within 40 or 60 feet of the surface.