1441 
were entirely due to the influence upon the polonium of the con- 
densation of gases still present in the apparatus. 
Experiments made in Leiden in liquid hydrogen with a provisional 
apparatus have convinced us that one might get rid of the conden- 
sations completely, even with Jiquid hydrogen, by using a ionisation 
chamber filled with pure gaseous hydrogen and a side tube with 
charcoal, immersed in liquid hydrogen. 
Conclusions. 
All these experiments which unfortunately are not so complete 
as we could have wished, confirm the independence of the radiation 
from the temperature, over a larger range of temperatures than 
had heretofore been done. Moreover these experiments have brought 
to light sources of error which must be taken into account, if one 
wants to make very accurate measurements at low temperatures. 
Astronomy. — “The periodic change in the sea level at Helder, in 
connection with the periodic change in the latitude’. By Prof. 
H. G. v. p. SANDE BAKHUYZEN. 
At the meeting of the Academy in February 1894 I read a paper 
about the variation of the latitude, deduced from astronomical obser- 
vations, and added to this a determination of the change in the mean 
water level in consequence of the variation of the latitude. 
Roughly speaking, one may regard the variation of latitude, as 
consisting of two parts, a periodic variation which takes place in one 
year, probably due to meteorological influences, and a periodic varia- 
tion which takes place in about 431 days, which depends amongst 
other things upon the coefficient of elasticity of the earth, its resistance 
to change of shape. As a consequence of these changes of position 
of the axis of the earth oscillations of the same periods must take 
place in the mean sea level and if we eliminate the annual oscillation, 
the periodic variation of 43L days remains. 
For the determination of the latter variation, I had made use of the 
mean sea level during the different months of the years 1855—1892, 
taken by the tide gauge at Helder. The results attained then for the 
amplitude and the phase of the periodic variation confirmed the opinion 
that such variations actually existed in the water, but as the changes 
in question are very small, it was desirable to extend the investi- 
gation in order to increase the accuracy of the results. I resolved 
therefore to submit to the calculations all the tidal observations made 
