548 



NA TURE 



[April 9, 1903 



and the author then passes on to the laws of flow, as deter- 

 mined by the length, shape and number of the openings 

 between particles. In the mechanical analysis of soils, the 

 mean diameter of the grains is known as the effective size, 

 and is such that if all grains were of that diameter, the soil 

 would have the same transmission capacity that it actually 

 has. The effective size is determined from the dimensions 

 of the mesh of a sieve which will permit 10 per cent, of 

 the sample to pass through it, but will retain the other 90 

 per cent. That is, in any soil, 10 per cent, of the grains 

 are smaller than the effective size and 90 per cent, are 

 larger. It is remarked that the velocity of flow through 

 porous strata is much less than might at first be supposed. 

 In the sands of the Dakota formation, from which remark- 

 able artesian wells draw their supply, the flow does not 

 exceed a mile or two a year. 



Underground waters are divided into three principal 

 zones : — (1) The unsaturated zone, (2) the surface zone of 

 flow, and (3) the deeper zones of flow. The motion of water 

 in the unsaturated zone is essentially vertical — downward in 

 supplying the saturated sheet below, and upward in supply- 

 ing the surface evaporation and the requirements of vegeta- 

 tion by means of the capillary action of the soil during 

 rainless periods. 



The surface or upper zone of flow extends from the level of 

 the water table to the first impervious rock floor. The 

 deeper zones of flow are those that lie below the first im- 

 pervious stratum, and the direction and character of the 



Fig. 1. — Contour Map showing position of water table (continuous lines), 



supposed lines ol' motion of ground water (arrowed lines), and the 

 thalwegs or drainage lines (heavy lines). 



flow are usually quite independent of the surface topo- 

 graphy, being controlled by large regional and geologic 

 conditions. 



The author points out that the unit of the surface zone of 

 flow of ground waters is the river valley, and the rate and 

 direction of motion conform primarily to the slopes and 

 grades of the land surface. The underground flow, in fact, 

 follows the trend and direction of the surface drainage. 

 The water table has a slope which is essentially similar to 

 the slope of the surface of the ground, though less steep. 

 The motion of the underground seepage into the streams 

 and rivers is similar to the lines followed by the surface 

 drainage into the same streams. 



The lowest line of drainage of the valley is known tech- 

 nically as the thalweg. Topographically, it is a line upon 

 a contour map which is a natural water-course (Fig. i). 

 Beneath the thalweg there is usually a similar drainage line 

 for the underground current, in general coincident with the 

 thalweg. For other parts of the valley the actual lines of 

 motion of the underground water are represented by a set 

 of curves which cut the contour lines of the water table at 

 right angles. The similarity of the contours of the water 

 table to those of the land surface enables one to sketch 

 approximately the lines of underground seepage from a con- 

 tour map of the surface. For the most part the lines of 

 flow run into the surface streams or thalwegs, but between 

 A and B, and X and Y, there is indication of an underflow 

 or general movement in the direction of the surface streams 

 and independent of the same. 



NO. 1745, VOL. 67] 



These views are worthy of attentive consideration and 

 study in connection with the geological structure, for, as the 

 author justly remarks, they must not be taken too literally. 

 The surface topography is only one, and often not the most 

 important, element in the control of the underground 

 current. He points out how irregularities in the form of 

 the first impervious layer and the amount of rainfall will 

 influence the distribution and motion of the ground water. 

 He directs attention also to the fact that much ground 

 water returns to the surface in the form of seepage which 

 is more important, though less obvious, than the springs. 

 Much ground water, moreover, may not find its way immedi- 

 ately into open channels, but may even take a general course 

 down the thalweg and flow through coarse materials toward 

 the sea in large underground streams or moving sheets of 

 water. This underflow is well known in the Great Plains 

 of America, although the movement is excessively slow. 

 Sometimes the underflows appear to be independent of the 

 surface streams, as indicated by chemical analyses. 



The deep zones of flow and artesian wells are finally dis- 

 cussed by the author; he deals also with common dug wells 

 and the influence of pumping on contiguous wells, as well 

 as the mutual interference of artesian wells. H. B. W. 



LONDON FOG INQUIRY, 1901-02. 1 



TN November, 1901, the Meteorological Council appointed 

 Captain Carpenter, R.N., D.S.O., a member of the 

 council of the Royal Meteorological Society, to conduct 

 an inquiry into the occurrence and distribution of fog in 

 London, initiated, with the assistance of a grant from the 

 County Council, in response to requests for more detailed 

 forecasts of the occurrence of fog. Captain Carpenter 

 at once put himself into communication with Captain Wells, 

 R.N., the chief officer of the Metropolitan Fire Brigade, 

 and made arrangements for the systematic observation of 

 fogs at some of the river stations and at other stations of 

 the Metropolitan Fire Brigade. He also arranged for 

 supplementary observations to be taken at certain of the 

 Metropolitan Police stations, at Battersea Park and Regent's 

 Park, at a number of coast-guard stations in the Thames 

 estuary, and by one or two private persons. Observ- 

 ations of temperature and other meteorological conditions 

 were obtained from a number of the stations and from the 

 parks ; self-recording thermometers were installed on the 

 Victoria Tower at Westminster, the Golden Gallery at St. 

 Paul's, on the roof of the Meteorological Office and at a 

 private house at Banstead. Regular records of fog in 

 accordance with a conventional scale distinguishing the kind 

 and intensity of the fog were thus obtained from a series 

 of points in or round London. By arrangement with Captain 

 Wells, special observations were made during fog or when 

 fog was anticipated by the forecast branch of the Meteor- 

 ological Office. 



Attention may be called to the following points in Captain 

 Carpenter's report, which is now issued : — 



(1) The first result of the inquiry is the suggestion of a 

 scale of fog intensity, arranged according to the inter- 

 ference with traffic upon road, rail, river, or sea, and re- 

 presented by the serial numbers o to 5. 



(2) Next it appears that on account of smoke the extreme 

 limit of visibility in winter from an elevated position in 

 London, in most favourable circumstances, is set at 1^ 

 miles. That limit is diminished as the tendency to form 

 fog is developed until the well-known effects of dense fog 

 are reached. 



(3) No evidence has been obtained of any special con- 

 nection between fogs and geological conditions. 



(4) The commencement of a fog is not identified with 

 any particular locality ; it seems to be a general process 

 depending upon general atmospheric conditions. There is 

 no evidence that fogs formed outside invade or drift into 

 London. The London fogs are produced in London ; they 

 do not come from the country. 



(5) The meteorological conditions for the formation of 

 fog are set forth and illustrated by charts and diagrams. 

 An interesting point brought out is a tendency to indraught 



1 Report to the Meteorological Council by Captain Alfred Carpenter, 

 K.N., D.S.O. 



