547 



On the contrary the protection against vibrations of long period 

 was vevy bad. Experiments with the suspended torsion balance 

 soon taught that, even during the most quiet hours of the night, 

 the torsion rod was never at rest. Sometimes the amplitude of the 

 oscillations gradually diminished to zero, but then the amplitude 

 increased again to 5 m.m., not to mention the extremely annoying 

 nutations of the mirror, which, indeed, never ceased. Apparently 

 vibrations of a period of 300 or 400 seconds (the period of the 

 torsion balance) are never failing in the marshy land of Amsterdam, 

 at least in the neighbourhood of the physical Laboratory. 



It was therefore hopeless to work with the torsion balance in 

 Amsterdam, and 1 resolved to continue my experiments in the cellar 

 of a country house near Huis ter Heide (prov. Utrecht). 



To my surprise I found that the stability of the balance, at the 

 new station, was most excellent. The motion of the mirror, about a 

 horizontal axis, was entirely absent and the amplitude of the oscil- 

 lations always decreased with time. After about one hour the image 

 seen in the telescope was at rest. The apparatus was placed upon 

 a wooden table, resting on the cellar floor. Even hard stampings 

 upon the floor in the neighbourhood of the apparatus had not the 

 slightest effect. 



Of course the temperature of the cellar v^^as very constant. One 

 disturbance had an effect on the observations, viz. the magnetic 

 action due to the iron beams of the cellar vault. Tlie constant 

 displacement of 0,3 m.m., noticed in the experiments with quartz 

 and recorded later on, is probably due to this cause. 



In view of the accuracy aimed at in the experiments, this amount 

 could not be neglected, though in some experiments its influence is 

 elin\inated. I therefore transferred the apparatus, first to a second 

 place in the cellar, where presumably the perturbations would 

 be less. 



Afterwards the apparatus was placed in the vestibule. Also here 

 the stability was excellent, but of course the constancy of tempera- 

 ture, though satisfactory, somewhat less. Several excellent series of 

 observations were obtained. As they extended, however, over the 

 whole day and the principal entrance of the house was then put 

 out of use, 1 restricted these observations to a rather limited 

 number of days. 



6. Vox- Amsterdam the latitude (f = 52,4 and g = 981,3 cm/sec 

 The angle a between the attraction of the earth b, and the resultant 

 of attraction and centrifugal force a, becomes 5'42" = 342". 



