TRANSACTIONS OF SECTION A. 543 



two millimetres apart, directly over the actinium salt. These two plates were 

 maintained at a difference of potential of about 250 volts. 



Measurements were made on the amount of the active deposit obtained at 

 different pressures on a narrow strip of the negatively charged plate, at a distance 

 x from the salt. The deposit was found (with different values selected for x) 

 to reach a maximum at a certain critical pressure which varied according to the 

 law p.x=a constant. From the fact that the maximum deposit followed this 

 law, it was concluded that the amount of active deposit corresponding to a given 

 constant amount of emanation under equilibrium conditions which possessed 

 a positive charge varied with the pressure of the gas in the emanation chamber. 



The expression I = — j — t— j- was deduced for the amount of active deposit 



possessing a positive charge under these conditions. It was also pointed out 

 that these experiments seemed to show that the deposit particles acquired their 

 positive charges from the surrounding ionised gas by a process of diffusion 

 rather than as a direct result of the expulsion of alpha particles accompanied 

 by electrons in the process of disintegration. 



8. Report of the Committee on Electrical Standards. — See Reports, p. 38. 



Department op Cosmical Physics and Astronomy. 



The following Papers were read : — 



1. On Barometric Waves of Short Period. By Dr. Wilhelm Schmidt. 



To carry out an investigation of barometric waves of short period it is 

 essential to eliminate the changes of longer duration as far as possible. In the 

 variograph a recording instrument with this air was shown. It registers, 

 instead of the barometric pressure at every moment, the rate of its variation— 

 i.e., its first differential coefficient in regard to the time. A uniform rise in 

 barometric pressure of ,-J-^ of a mm. mercury per second, for instance, is in this 

 instrument recorded by a constant deflection of the recording pen of 1 to 3 cm. 

 from the zero-line. 1 



A good place for researches of this sort was found at Innsbruck, in the 

 Tyrol. Here, especially in winter time, exist all the conditions necessary for 

 the formation of a cold-air lake at the bottom of the valley, whereas above 

 it there is often a warm, dry, southerly wind, the ' f oehn. ' By its influence waves 

 are generated in the separating surface and recorded at the bottom as barometric 

 oscillations, the pressure being higher under the crest of a wave, lower under 

 its valley. 



Samples of records were shown, obtained at two points, 1,950 metres apart. 

 They led to the conclusion that waves of longer duration are probably standing 

 waves, while those of shorter period are propagating ones. In a certain case 

 of foehn, where movement and temperature of both layers could be deduced from 

 observations, a comparison with the theory was tried. 



The formula of Helmholtz gives for the duration of a single oscillation a 

 value between 3 min. 17 sec. and 3 min. 19 sec. if one takes into account the 

 current of the foehn only — a minimum value of 36 min. with the observed 

 westerly wind at the bottom. Both sorts of waves are independent of one 

 another, and, if formed, there will be only a sort of interference on the variograph 

 curves. The observed period was 35 min., which is in very good agreement 

 with the theory. Standing waves would also be possible, due to transversal 

 oscillations in the cold-air lake, but with a probable duration of 9 min. about. 



As the conditions here are very favourable, regular waves of pressure were 

 found very often in the mountain- valley. But they occurred also at. a place 

 with much freer exposure, at Vienna. Here they could not be so dependent 

 upon local conditions, but must show the influence of the general conditions 



1 The instrument is fully described in Wiener Sitzungsberkhte, June 1909. 



x n a 



