452 Intelligence and Miscellaneous Articles. 



a higher degree of rarefaction, and therefore at an increased resist- 

 ance, in which the apparent attraction no longer occurs. 



A conductor also which does not encircle the one carrying the 

 current partially exerts a screening action, and to a greater extent 

 the longer it is. Prof. Hertz has shown that a system of parallel 

 stretched wires may also act as a plane-shaped conductor. Such 

 a system of wires can, however, only exert a screening action if 

 the primary current can induce currents in those wires ; such 

 wires form, therefore, no screen when they are placed at right 

 angles to the primary conductor. Induced currents form also an 

 essential condition that a conductor can reflect inducing actions. 

 The reflected actions are the actions of the currents which are 

 induced on the surface of the reflecting conductor. — Wiener Berichte, 

 January 16, 1890. 



ON THE OSCILLATIONS OF PERIODICALLY HEATED AIR. 

 BY DR. MARGULES. 



Prof. Hann's investigations on the Daily Oscillation of the 

 Barometer led the author to calculate the variations in pressure 

 which result from periodical changes of temperature in the air. 

 Plane waves of temperature progressing in a plane stratum of air 

 produce pressure- waves of equal period, the amplitude of which is 

 greater the nearer the velocity of propagation of the constrained ap- 

 proximates to that of the free vibrations. If the atmosphere were cut 

 into a great number of zones each of which was traversed by a daily 

 wave of temperature, the zones near the equator would have waves of 

 pressure in which the maximum coincided with the maximum of 

 temperature ; near the poles the phases would be opposite, at 50° of 

 latitude the amplitude would be very great, and in two adjacent zones 

 of opposite phase. The magnitude of the differences of pressure be- 

 tween two separate zones which thus results shows that the transfer- 

 ence to annular spaces of the calculation which holds for plane waves 

 is not sufficient, and that the movements of the air upon the globe 

 must be calculated without diaphragms, if waves of temperature 

 travel from meridian to meridian. The calculation for the spheroid 

 at rest shows, as already demonstrated by Lord Eayleigh in a 

 recent paper (Phil. Mag. February 1890), that the semidiurnal 

 waves of pressure on the earth are far smaller than the diurnal 

 ones, even if the corresponding waves of temperature are of the 

 same amplitude. But the semidiurnal variations of temperature 

 (which are obtained by decomposing the daily curve of temperature 

 by the method of periodic series) are small compared with the 

 diurnal ones. How, notwithstanding this, the relatively large semi- 

 diurnal variations of pressure may be explained, has incidentally 

 been indicated by Sir W. Thomson. The calculation for the rota- 

 ting spherical shell, made in a manner analogous to that for La- 

 place's calculation of ebb and flow, confirms Thomson's supposition. 

 Small semidiurnal variations of temperature are sufficient, with a 

 corresponding choice of the mean temperature, to produce very 

 great variations of pressure. — Wiener Berichte, March 6, 1890. 



