Mr. W. 11. Birt on Atmospheric Waves. 125 



been supposed to be manifested by Aristarchus. Under the 

 illumination of the earthlight this is by far the brightest object 

 on the dark part of the moon^s face^ and is visible much longer 

 and with poorer glasses than any other object there. 



Truly yours, 



Harvard University, Cambridge, ^'' ^' ^HALER. 



Massachusetts, U.S.A. 



XVII. A Contribution to our knowledge of Atmospheric Waves. 

 By W. R. BiRT, RR.A,S., F.M.S.^ 



THE issue of weather-charts by the Meteorological Office 

 places at the disposal of such amateurs as may be willing 

 to subscribe for them very valuable data bearing on the general 

 affections of the atmosphere, temperature, pressure, wind, &c. 

 Having given some little attention in former years to the subject 

 of atmospheric waves, I have looked into the weather reports and 

 charts for evidence bearing on these movements, and have suc- 

 ceeded in finding some important corroborations of the views I 

 announced more than a quarter of a century ago, as well as 

 some of a novel nature which the data then in my possession 

 were unable to elucidate. 



In the interval that has elapsed between the present and my 

 former researches, a most important generalization has been 

 arrived at ; it is that, whatever may be the direction of the wind, 

 the region of lowest pressure is on the left of this direction, and 

 the region of highest pressure on the right. Observation shows 

 that winds are arranged in two ways — cyclonic and linear; and 

 in both, this law, known as Buys Ballot's law, holds good. If 

 any wind of a linear arrangement be considered in relation to 

 pressure, places on the left of the line of this wind will have 

 lower pressures, with either the same wind or winds appertaining 

 to the slope of a wave coexistent with that of which it forms a 

 part, but which is moving in a different direction ; and places on 

 the right of the line of wind will have higher pressures, also with 

 the same wind or other winds, as in the case just alluded to. 

 The region between the lines of highest and lowest pressures is 

 considered as being under the slope of an atmospheric wave (see 

 figs. 1 and 2). 



Between a true linear and a t7'ue cyclonic arrangement of wind 

 every variety of curvilinear motion, from a very slight to a very 

 decided curve, may exist. By the air flowing into a cyclone in a 

 direction contrary to that of the hands of a watch the upward 

 movement is continually fed, while the outflow from a region of 

 high pressure is in the opposite direction, or with the hands of a 

 * Communicated by the Author. 



