( 754 ) 



■\vas then about 0.34 M. But tlie tide rose then unusuallv high 

 (0.55 M. above the average high-tide mark) the low-tide mark being 

 then just the axerage one. I thinly I may estimate the average \ ertical 

 amplitude in the well to be, at the most, 0.30 31., and believe pretty 

 near to hit it. when accepting 1.30 M. -|- A. P. as the average level 

 in that well, «u' 1.50 M. above the average sea-level. 



When considering the motion of the deep ground-water, in the 

 direction of the sea, caused by the hydraulic pression in the dunes, we 

 must not overlook the much greater specific weight of sea-water. A 

 column of sea- water 30 M. deep, (with a specific weight of 1.0244, on the 

 average, as that of the Xorth-sea-water), will be kept in balance by a 

 column of fresh water, about 0.75 M. higher. Xo doubt however the 

 depth of the fresh ^^■afer, in the coarse-grained sand below it, is much 

 greater than 30 M. :- A. P., without any considerable decrease in the 

 ascending power. A direct proof of this is the small percentage of 

 chlorine of the water in the deep well in the Kerkplein, amounting 

 only to 45 ni.G. p. Liter, and that in the well on the beach, 

 30 M. deep, about 250 ]M. more south, ^^•as 52 m.Cr. the Liter. In 

 statistic equilibrium, 1.50 M. above the average-level of the fresh under- 

 ground water would corresjiond with a depth of 61.5 M. But on 

 account of the motion of the fre>h water we have here to deal \\ ith 

 a condition of dy nauiic e(|uilibriuni : the pressure at great depths 

 consequently is not siini>ly settled by the height, to which the 

 water ascends higher up, in the ground. However below 30 M. (in 

 the coarse-grained sand) there will be little difference, so we cannot 

 but accept, that an extra-pression of 1.50 M. of the sweet ground- 

 water, apparent from the level the water reached in the tul)e, will 

 correspond with a depth of about 60 M. -r A. P. ; taking in con- 

 sideration the decrease of pression downward, Ave may safely state 

 the depth to be 50 a 60 M. -i- A. P. One thing is sure, the water 

 wliich rises from al)out 30 M. -^ A. P., has still ascending power 

 above the level of the .sea. This may be distinctly observed in the 

 well, sunk on the beach, tilthough being 300 M. nearer to the 

 low-tide line, a considerable decrease can be noticed. At that small 

 distance, the deep underground water in the dunes has already, for 

 the greater part, lost its ascending power and we may accept that 

 not far out into the sea, it is entirely gone. That strong pression-fall 

 must l)e principally attributed to tide-fluctuation, which every time 

 renews a fourth of the water in the sand, as far as that fluctuation 

 makes itself felt; apparently (the well in the Kerkplein shows it) at 

 a rather considerable distance, from the sea. But also the fact that 

 the beach slopes down — at Zandvoort the depth of the sea, 400 M, 



