(70) 
hand a very marked systematic difference existed between the means 
for the half-years January 
Mr. WiLrTERDINK found 
June and July—December. 
Obs.—Comp. 
1886 January—June + 0°.045 
July—December — 0.08 
1887 January—June + 0.035. 
Consequently, when in 1890 I undertook the definitive discussion of 
the time-determinations and clock-rates for the period 1877— 1882, it 
appeared desirable to me to investigate whether a similar phenomenon 
would again show itself. I then found that the years 1878—1882 
(before May 1878 the rate was not yet sufficiently regular) were in 
this respect entirely similar to 1886—87, and, investigating the pheno- 
menon more closely, | moreover found that the rates, after correction for 
temperature, still showed a yearly periodicity which accordingly 
had its maxima in the months of equal temperatures April and October 
and the amplitude of which was about O0°.10. 
1 then continued my investigation (the results of which were briefly 
published in the # Verslag van den staat der sterrenwacht te Leiden 
1889—1890,” pages 14—15) in the same direction and included on 
the one side the years 1882 
90, using provisional results of the 
time-determinations, and on the other side the years 1862—1864, 
using the results of Kaisers investigation in Astr. Vachr. 1502. For 
these two periods I also found the same unexplained inequality. 
After 1890 this investigation was abandoned for the time being, 
and it was only taken up again last year. In the mean time the 
clock had been removed to its new position, and it now appeared that, 
notwithstanding the fact that the changes of temperature had become 
much more gradual, still the yearly periodicity of the rate showed 
itself in the same way as before, and its deviation from the yearly 
periodicity of the temperature was certainly not less evident’). 
It became thus evident that there was no question of accidental 
circumstances, which could be altered by cleaning the clock, nor of 
such conditions as depend on the special nature of the changes of 
temperature to which the clock is subjected, but that the cause of 
the phenomenon must lie deeper. 
It therefore appeared desirable to subject the way in which it 
showed itself in the three periods (viz: before and after the cleaning 
1) See also „Verslag van den staat der sterrenwacht te Leiden 1898—1900 pages 
12—13. 
