( 47 ) 



VAN DiKSKN is found ill the Vci-sl. (mi Moded. dor Koii. vVkad. 1885^). 

 I can by no means accept the amount found tliere. The cliief reason 

 why Mr. van Diesen arrived at an erroneous fioure is tliat lie 

 assumes that the groundwater in the Ilaarleinmerineer polder which 

 is situated at a depth of about a metre below I he surface evaporates 

 as much as water at the surface on account of the iutersl ices between 

 the particles of ground. This, 1 think, is entirely wrong; ground- 

 water at a de[)th of a metre does not evaporate at all. 



Now if we assume that the groundwater does not evai)orate, the 

 figures given in the paper would lead to a negalivc afflux, which 

 certainly is wrong too. This is a consequence of another reason 

 why an erroneous figure is found, namely the method of derivation. 

 Mr. VAN Diesen, namely, calculates the amount of the afflux from 

 two periods of six years, for each of which he derives the equation: 



/• =z (t,i; — /j 

 in which /• is the amount of the flow, ,/- the ratio of the evaporation 

 at the surface and the rain fallen ; /i and /> constants, deri\ ed from 



the other data. So he has two equations /• = rnj — and k^ =^ (t^r^ A . 



Now he determines the ratio of Z' and l\ from the difference in level 

 of the water in the "bosom" ') and of the polderwater, a ratio 

 naturally little differing from unity ; he further assumes that the ,/• 

 of each period has the same average \alue, so he puts .n = ,v . The 

 two unknown quantities, k and d', can then be found from these two 

 equations. 



But ,v and ,i\ are not exactly equal. Mr. van Diesen himself says: 

 "evidently this value must change according to circumstances." If 

 -I'l be equal to ,/; -f- L we have : 



I k 



—b^ — b —«1 L 



. ^'i ^^1 



''' ~" /; k 



— «J — a — a J — a 



k 

 Now it will entirely depend on the value of — or, and a whether 



L has an appreciable influence on the value of ,/■. Since from the 



/.: 



nature of the case — r/, and a are great values, differing little between 



/c, 



each other, however, a small value of L has a great influence on 

 X and. hence on /'. 



1) Versl. en Meded. van de Kon. Akademie van Wetenschappen, Aid. Nalnnrk. 

 8e Reeks, Dl. I, p. 359—374. 



2) "Bosom" is called an intermediate discharge canal or Ijasin. 



