[13] OYSTER CULTIVATION IN THE NETHERLANDS. 1013 
enough not to allow any considerable quantity of sea water from higher 
layers to flow into it. 
In conclusion, we must mention that through Mr. W. Leemans, engi- 
neer, we obtained from the ministry of public works the loan of an iron 
areometer, which was subjected to a comparison with the above-men- 
tioned instrume ts, and which was used in the observations made at 
Tholeu, of which we shall speak later. 
We also employed a number of thermometers and areometers from 
the firm of Kipp, at Delft, likewise with the view of obtaining a knowl- 
edge of the instruments most suitable for our purpose. 
This will suffice to give an idea of the instruments with which we oper- 
ated, and we will only briefly refer to the method pursued in our opera- 
tions during the past summer. 
Thanks to the friendly assistance of the minister of marine, we got 
the promise of co-operating with us from Mr. J. F. van Kervel, retired 
lieutenant, Ist class, Netherlands Navy, and during the course of the 
summer we had ample opportunity to experience the great value of his 
services. From his former occupation, more familiar than any of our 
number with the use of physical and meteorological instruments, he was 
ever ready not only to give his advice regarding the most profitable way 
of making observations and the conclusions which might be drawn from 
such observations, but also to give his personal aid whenever he thought 
that his presence at any of our stations of observation might be of use 
in furthering our object. Also in August, when, through the kindness 
of the minister of war, we obtained the use of one of the steamers of 
the torpedo service at Brielle for two weeks for the purpose of making 
observations on the Hastern Scheldt (see the private report on the sub- 
ject), it was Mr. van Kervel who, in spite of the extremely unfavorable 
weather, gave his personal aid and attention to these observations. We 
herewith express to him our warmest thanks, and the hope that in the 
future we may count on his co-operation, without which our progress on 
this (to us) strange field would be considerably delayed, if not entirely 
stopped. 
The method and object of the last-mentioned observations were as 
follows: 
(1.) To ascertain to what modifications the temperature and saltness 
of the sea water in the Eastern Scheldt is subject: 
a. At different times. 
b. At different depths. 
(2.) To ascertain if these modifications are everywhere apparent to 
the same degree, and also if there is a noticeable difference in this re- 
spect between banks which are deep under the water and those which 
are occasionally dry. 
(3.) To ascertain whether there is any connection between these modi- 
fications and the phenomena of the current, as far as they have been 
observed by the engineers of the Department of Public Works. 
