The magnetic storm between April 24th and May 6th was accompanied by a decrease of cosmic ray intensity at Amsterdam of maximum 5%. At ~, = 54 ยฐ only corpuscles of an energy lower than 8.10 ~ eV. were influenced. In some cases but not in all of them a parallelism occurs between the variation of intensity and the variation of H. The decrease of the C.R. inte~asity can be explained by a decrease of the normal intensity of the circular currents around the earth.
Shortly after the magnetic storm at the end of April 1937 we inspected our records of the CR intensity and found that there was only a small variation of the ionisation values in our vessels shielded by 110 cm Fe, in agreement with the barometer variation at the same time. At that moment the record of the vessel under 12 cm Fe was not yet Worked out and our attention was not drawn to the present question, until we had read the publication of F o r b u s h 1) and H e s s and D e m m e 1 m a i r ~), whereupon the records of the three instruments with which we obtain our hour-records, were worked out. All three instruments had an ionisation chamber of 40 L filled with argon at 45 arm. and the accuracy of the record was certainly not less than 1 ยฐ/00. The results are given in fig. 1 together with the barometer record and the values of the Hor. and the Vert. 'component of the earthmagnetic force.
First, the barometer correction was applied. The barometer coefficient was found from the values obtained on various occasions. For instrument 1 it was 1,7%, for instrument 2 it was 2,2% and for instrument 3, per cm Hg, it was 2,60 *).
*) It is very remarkable that we invariably found the coeff, of instrument 1 smaller than that of instrument 2, although the layer above the two instruments was the same. The only difference was that the layer of iron was under 2 twice as thick as under 1. After having interchanged the positions of the two instruments, the barometer eoeff, of --111 --