^8 PROCEEDINGS OF THE CANADIAN INSTITUTE. 



length." He also states that " waves of smaller and smallest wave-length are theo- 

 retically possible." 



His theory is borne out by observations taken at the Blue Hill Observatory, 

 near Boston, by Mr. Clayton, who has found that the larger waves, as marked upon 

 the barograph traces, have a maximum frequency with northeasterly winds, and a 

 minimum frequency when the wind is from the south-west, and also that the greatest 

 number occur during the winter months. 



Too much importance cannot be placed upon the above statements, as they are 

 likely to prove of inestimable value in helping us to a better knowledge of the 

 mysterious forces at work in the upper atmosphere. 



In the accompanying diagram the upper, or poleward current, is represented as 

 moving approximately from the south-west to the north-east, in its spiral course 

 around the globe. Its average velocity is 60 miles per hour in summer, which 

 increases to 112 miles per hour during the winter months. 



The heavy lines indicate the lower stratum of air as travelling in an opposite 

 direction to the upper current, which would be the case during the approach of a 

 storm from the south-west. At the upper surface of this lower stratum huge 

 atmospheric waves, or billows, are set up, due to the rapidly moving, opposing, 

 upper poleward current of a lesser density. The influence of these huge waves 

 often extends to the earth's surface, where they have been recorded upon barograph 

 traces. The lighter lines represent a second form of wave movement in the lower 

 stratum of air (say cumuli level), caused by the two subsidiary strata of the lower 

 air travelling at different velocities and directions to one another, as is often 

 observed during the approach of an important storm centre. These waves also 

 extend to the earth's surface, where the larger ones are found recorded upon the 

 barograph traces as short and rapid oscillations. These are well shown upon the 

 Observatory photographic trace, now before us (Fig. 2), which is a record during 



I^ZIZ^Z/^^ J,^ :— 



JD^nn 



the approach of a storm from the south-west, upon which you will observe the 

 larger undulations begin before the barometer falls, and are evidently caused by 

 huge billows, set up at the boundary surfaces of the upper poleward current and 

 the lower air straturn, which, owing to the position of the storm centre, would be 

 rapidly moving in opposite directions. As the storm approached the undulations 

 become more rapid, with a marked time interval of 10 minutes. These appear to 

 correspond with the waves set up along the still lower, or cumulus level. As well 

 as from the barometric record, the existence of these atmospheric waves, under 

 certain conditions, may often be observed in the formation of the clouds above us; 

 for instance, during fine, anti-cyclonic weather, one has frequently noticed great 

 parallel bands of cirri appear in the west, and rapidly extend eastward, in advance 

 of a cyclonic area, then over the Southern States. These represent the crests of the 

 larger billows mentioned by Helmholtz, and are caused by the lower, denser stratum 

 of air being forced up into a lighter and colder level, where condensation takes 

 place. The following illustrations plainly demonstrate the existence of air waves 

 found along the cumulus level. These forms, no doubt, are familiar to all present, 



