30 PROCEEDINGS OF THE CANADIAN INSTITUTE. 



From the tabulated data and the other tidal curves, studied in connection with 

 the bi-daily weather charts, which bear out the increase of oscillations preceding 

 the approach of atmospheric depressions, it appears that these secondary undulations 

 are due to atmospheric waves or billows set up in the upper atmosphere. 



We are told by the late Professor Von Helmholtz, (4) who has made a special 

 study of these waves from theory and analogy with ocean waves, and has calculated 

 their possible forms and dimensions, that "As soon as a lighter fluid lies above a 

 denser one, with well-defined boundary, then, evidently, the conditions exist at) 

 this boundary for the origin and regular propagation of waves, such as we are 

 familiar with on the surface of water. This case of waves, as ordinarily observed on 

 the boundary surfaces between water and air, is only to be distinguished from the 

 system of waves that may exist between different strata of air, in that in the former 

 the difference of density of the two fluids is much greater than in the latter case. 

 Since the moderate winds that occur on the surface of the earth often cause water- 

 waves of a metre in length, therefore, the same winds, acting upon strata of air, say 

 10 degrees difference in temperature, maintain waves of from two to five kilometres 

 in length. Larger ocean waves, from five to ten metres long, would correspond to 

 atmospheric waves of from fifteen to thirty kilometres, such as would cover the 

 whole sky of the observer." 



Mr. Clayton, of the Blue Hill (Observatory (5) has graphically shown the 

 coincidence and easterly progression of the larger atmospheric waves by means of 

 tlie daily synchronous barograph traces from stations south and east of his, plotted 

 upon the same time sheet, and has also shown that the maximum number of waves 

 occur during a northeast wind, and the minimum when the wind is from the south- 

 west, and that the greatest number were recorded during the winter months. 



After referring to the above valuable investigations by two such well-known 

 scientists, let us return to the foregoing table and frame a few leading questions, 

 Vv'ith the answers which appear to me most satisfactory. Further discussion will 

 doubtless throw more light upon this subject. 



1. Why do the secondary undulations become decidedly marked from noon of 

 the 5th, although the barometer is steadily rising, and the wind at the earth's 

 surface light or calm ? 



Because the upper part of the lower stratum of air, not being retarded by 

 friction due to contact with the earth, and of greater specific gravity than that to 

 the south and west of it, begins to move towards the region of lighter air, viz.: in 

 a southwesterly direction, the barometric gradient being small, the movement is 

 slow, but being in an opposite direction to the upper stratum, which is less dense, 

 and rapidly moving polewards (mean winter rate, 112 miles per hour), (6) but 

 inclined to the east. According to Helmholtz, this should be sufficient to produce 

 along the boundary surfaces of the two strata, waves which may extend to the earth. 



2. From 8:00 p.m., after the storm centre had passed to the north of Halifax, 

 and a moderate gale still blowing from the northwest, why do the rapid undulations 

 decrease until they almost form an unbroken curve by midnight ? 



The wind being westerly, a decided decrease of the undulations should be 

 expected, on account of the lower stratum of the air, now moving approximately in 

 the same direction as the upper layer, and as the force at the surface of the earth 

 is still that of a moderate gale, greater velocities may be looked for at higher levels, 

 where friction (viscosity not included), caused by contact with the earth, is 

 inappreciable, thereby tending to equalize the easterly rate of movement of both 

 strata; also well-defined boundary surfaces are not so likely to exist, owing to the 

 more uniform temperature throughout both strata. 



(4) Paoerread before the Prussian Academy of Sciences at Berlin, on July 25th, i88g, and translated in 

 the Smithsonian Reports by Professor Abbe. 



(5) Blue Hill, Mass. Meteorological Observations. Vol. XL., Part III., Appendix E., 1893. 



(6) From Blue Hill cloud observations. 



