TliANSACTlONS OF tfECTIO>* A; 600 



stations, viz., Cambridge, South Kensington, Westminster, Siiepherd's Busli, and 

 Leighton Park, Reading, were exhibited (see Plate VII., June 30). Another, 

 Petersfield (Ditcham Park), has been added. The distance apart between Cambridge 

 and Petersfield is about 180 miles. The record for the hour 5 a.m. to 6 a.m. for 

 each place is reproduced. There is barely a quarter of an hour shown between 

 the occurrence of the trough of the first wave at any of the places. No great 

 stress can be laid on the accuracy of timing, and the South Kensington record in 

 particular appears to be mistimed. In any case, it would seem that the dis- 

 turbance, if not simultaneous at the different places, travelled fa.<ter than 109 

 miles per hour.* 



One other example of very regular oscillations was shown, namely, that of the 

 photographic record of the Falmouth barogram for December io, 1900 (see 

 Plate VII.). The undulations, in this case also subject to damping, are clearly 

 shown in the reproduction. 



The problem presented by these examples was to determine whether the 

 oscillations were due to the vibrations of the atmosphere as a whole or in part, 

 and, in the latter case, to specify w^hat part. It was necessary again, as the late 

 Lord Salisbury said about the luminiferous ether at Oxford, to tind a nominative 

 case to the verb to undulate. 



Professor Lamb had suggested only gravitational and vertical wave-motion 

 in an atmosphere stratified in discontinuous layers. The question was asked 

 whether it was possible that a steady atmospheric current of wind, with its 

 transverse pressure gradient and its great momentum, could take on an elasticity 

 for transverse vibrations in consequence of the quasi-rigidity of its motion relative 

 to the earth's axis, and thus form a system capable of horizontal oscillations with 

 associated pressure changes. 



Professor C. H. Lees pointed out that since Dr. Shaw's observations .showed 

 that when there was a sudden change of atmospheric pressure it was at times 

 followed by an oscillation of pressure, and at other times not, it looked as if we 

 were dealing with an oscillatory system in which the damping term due to 

 viscosity had nearly the critical value, and in which, therefore, a small chanue of 

 viscosity would lead on the one hand to oscillation, on the other to no oscillation. 



Prol'essor A. Laavrence Kotch, Director of Blue Hill Observatory, U.S.A., 

 said that the waves set up in the atmosphere during thunderstorms were of com- 

 mon occurrence at Blue Hill. But another class of waves, arising from different 

 conditions, was discussed by Mr, H. H. Clayton in vol. xl. part iii. of the 'Annals 

 of the Astronomical Observatory of Harvard College.' These waves occur to the 

 north-eastward of a cyclonic .storm moving north-east, and the same wave has 

 been traced on the barographs of stations situated several hundred miles apart, 

 the velocity of propagation of the wave from west to east being somewhat oreater 

 than the motion of the cyclone centre itself. As a rule, oscillations iu wind 

 velocity and direction are .synchronous with the barometric waves, the wind 

 reaching a maximum velocity from a northerly direction during the passan-e of 

 the cre,st of the wave and a maximum velocity from an easterly direction in the 

 trough of the wave. Nearly all the waves occur in rainy weather, and usuallv 

 there is an increase in the intensity of the rainfall at the waveorost. That 

 similar waves occasionally exist in the upper air is shown by the undulations 

 seen in clouds. According to a study of special cloud-forms at Blue Ilill by tlie 

 late Mr. A. E. Sweetland, published in the ' Annals of the Astronomical Observa- 

 tory of Harvard College,' vol. xlii. part i., the greatest number of undulations 

 are observed in the cirrus and cumulus levels. At the lower level they occur to 

 the north-east of the cyclone centre and precede warmer weather, beino- due to 

 a warm current flowing over a colder current near the ground, a condition which 

 Von Helmholtz has shown to be necessary to the formation of undulatory clouds 



' In considering these and the other traces of the microbarograph, it should 

 be remembered that the recording pen is carried by a lever and describes a circle 

 In some of the traces the effect of the curvature of the ordinates is very marked 

 and in the Shepherd's Bush record of June 30 there is a gradual rise of the poa 

 during the period attributable to temperature change. 



1908. R K 



