( ^^J ) 



east of tliis dmiiiauc a layer of fresh- water of at least 50 M. iliick 

 may have aeeunmlaled ? This means a yearly iiierease of at least 

 one metre, or, if we take into account the interstices between the 

 grains of sand, of about O.'M) M. pm-c water a year. Now this 

 amount is i)retty much the same as of the rain that can |)enetrale 

 into the earth, while also ail the other siirfacc-walcr can furnish 

 fresh-water to the underground, by which also the fresh-water, 

 llown otf to ilie deep pohkM-, can be acc(»uiil(Ml for. hi u\\ former 

 communication 1 already |>oin(ed out thai the |>o\\(>r of chi\- lo 

 ti-ansmit water is commonly underraled. The clay in oui- allu\ial 

 grounds is generally very im})ure, consists mostly <»f xerv lijie sand 

 and according as the percentage of this incivases, its "j)ermeabilitv" 

 becomes greater. The fattest clay of the Kiekei- jxilder at Sloten lies 

 as a thin bank immediately njider the su[)erlicial layer of fen, and 

 contains :307, real clay. From the 7 lAI. "clay" iji the Rieker polder, 

 one has to subtract tirst a couple of metres of sand, the rest is also 

 much richer in sand than the fat u|)i)er layer mentioned. Now 

 Spkino has proved that a layer of Hesbay's loam of 7 M. thickness 

 admitted in 24 hours a movement of water of at least ().()o(> to 

 ().()45 M. ^) which is ten to fifteen times more than the velocitv 

 calculated for the Rieker [)older. A sample of loam, kindly sent me 

 by that scientist, proved, on analysis by Dr. N. Sciiookl, to contain 

 ■2i.5"/„ clay i.e. about as much as our ordinary, pretty fat alluvial 

 clay contains on an a\ erage. E.xperimeids with fatter clav ujider 

 [)ressure, as it is in nature, give me a much smaller velocity which 

 however is still sutïicient to explain the li\ drological coiidition of 

 the Rieker polder. Of these ex[)erinients I iuteiid to give an account 

 on a future occasion. 



I wish to draw attention to a residt of the exjx'riinents ofSi'inNc; 

 already mentioned in my former communication, according to which, 

 when the thickness of a layer of sand becomes very great with 

 respect to the pressure-column of the water, the i-ate of tilti-ation 

 may by uu means be taken inxei-sely ]»roportional to the thickness 

 of the tilter. On the conti-ary, SpkiN(; found iu this case the rate 

 independent of the thickness of the filter. I can confirm this for 

 clay and for this substance the [)ressure may even be relativelv 

 great and the thickness of the layer hundreds of times smaller on 

 account of the so miu-h greatei' resistance of clay lliau of sand. A 

 layer of the fattest clay, obtained in the same way as the com- 

 pressed fen, by scpunv.ing out the ^\ater, having a Ihicknc'ss of 



1) In this timr namely a iayer of water 'if I'J to l.") mm. Iliicjsnc.-^s was tians- 

 milled. 



