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plants could flourish there. And such a state of things obtained 
in all the dunes, outside the influence of the large water works, 
arriving at a maximum about 1880. 
A few years later the present dry period commenced, by which, 
also independently of each artificial cause for a lowering of the ground- 
water in the dunes, its height decreased so much that in the present 
winter water is only found at 2 M. below the level surface of plains, 
which about 1880 were flooded in winter. Certainly no less than 
2 to 2'/, M., in some places probably more, the ground-water is 
now lower, by natural causes alone, than it was in those wet years. 
The periodical changes of the level of the ground-water in the 
dunes, thus appearing during more than a century, agree in striking 
conformity with the thirty-five-year period discovered by Prof. Ed. 
Brickner, according to which, in almost all the countries of the earth, the 
rainfall and the height of the water of lakes changes. Really between 
1786 and 1805 a dry period occurs (for the Netherlands too this 
appears from the rainfall as determined at Zwanenburg). So in the 
beginning of the 19 century a minimum of rain and the lowest 
watermarks were reached. Then follows an epoch of much rain 
between 1806 and 1825, again a dry period from 1826 till 1840, 
a new period of much rain from 1841 till 1855, a dry period from 
1856 till 1870, with a minimum about 1860, the latest period of 
much rain from 1871 till 1885, with a maximum about (880, finally 
again a dry period, with a minimum about the end of the nineteenth 
century. A few years ago we still were in this dry half of the cycle. 
Since, some years ago Dr. WirLiam J. S. Lockysr*) proved, that in 
the amount of spotted area of the sun also, a thirty-five-year period 
could be traced, from 1833 till 1900, the discovery of Brickner surely has 
‘still gained in importance. On the other hand we now understand better 
what may be the cause of the few temporary or lasting deviations 
of some countries. If the better insight we now have got in the 
cause of the phenomenon discovered by briickNer is well adapted to 
increase our confidence that we have to count with it in future, we 
also need not suffer ourselves to be prevented from this by the 
deviations in question. 
Lately, from the extensive study of the rainfall in Germany by 
Dr. G. Hettmann’) and its discussion by Brickner *), it again appeared 
how in our vicinity wet and dry periods, and pretty weli simultane- 
ously with those in the dunes of Holland, alternate with one another. 
1) Proceedings of the Royal Society. Vol. 68. (1901). p. 285—300. 
*) Die Niederschläge in den norddeutschen Stromgebieten. 3 Bande, Berlin, 1906, 
°) Meteorologische Zeitschrift. Wien 1906, p. 565. 
