38 REPORT — 1860. 



temperature are propagated from one place to another in mountain countries. Con- 

 siderable variations of temperature are not unfrequent, and sometimes occur very 

 rapidly, usually if not always in connexion with changes of wind ; but we know very 

 little of the way in which a disturbance of this kind is transmitted either in the 

 horizontal or the vertical direction. It is conceived that a network of observations 

 made by a considerable number of observers scattered over a district, such as Swit- 

 zerland and Piedmont, would lead to some increase of our knowledge in this respect. 

 4th. Observations on the temperature of the surface and upper layers of the soil 

 have a considerable bearing on many questions connected with the distribution of 

 plants. One difficulty in investigating these questions arises from the difficulty of 

 comparing observations not made upon a uniform plan. It is thought that the 

 adoption of uniform instruments, and a plan of observations previously agreed upon 

 by all the members of the party, will much increase the value of their results. All 

 the instruments used in these observations are exactly of uniform construction, and 

 made by Mr. Casella with the utmost practicable regard to lightness and convenience. 

 Each instrument is numbered for purposes of future reference. 



On Atmospheric Waves. By W. R. Birt, F.R.A.S. 



The object of this communication is rather elucidatory than otherwise. It is now 

 twelve years since I had the honour to lay before this A>sociation the last of my 

 reports on the subject. During the interval it has doubtless occupied the attention 

 of other minds, and some degree of misconception may have arisen which may call 

 for some elucidatory remarks on my part, especially as the series or reports in our 

 annual volumes has been referred to on the Continent, as establishing a priority of 

 investigation into these phenomena on the part of the British Association for the 

 Advancement of Science. 



It is now several years since Professor Dove announced as his conviction that the 

 equilibrium of the atmosphere was maintained in the extra-tropical zones, more by 

 parallel than superposed currents, that these currents had a shifting transverse or 

 lateral motion, and in consequence, so to speak, they advanced " sideways." I am 

 not aware that Professor Dove connected these shifting parallel currents with baro- 

 metric phenomena, although he did with thermometric. In the course of my inves- 

 tigations into those phenomena termed atmospheric waves, I ascertained, by carefully 

 discussing the records of the wind for the greater portion of November 1842, that 

 not only such parallel compensating currents existed as stated by the Professor, but 

 that during the period under inquiry, a similar system of parallel and compensating 

 winds were blowing and moving at right angles to them. The arrangement of these 

 cross winds was N.E. — S.W. and N.W. — S.E. I also found that these winds were 

 intimately connected with barometric pressure, so that when the barometric curve 

 •was projected and presented the wave form, the mind was led to group under the 

 general term " atmospheric wave," at least two if not three distinct classes of phe- 

 nomena. First, the winds succeeding one another, as we know they do with more or 

 less regularity. Second, the pressure, a more or less continuous fall of the barometer 

 generally succeeding a gradual and continuous rise : both these phenomena are 

 capable of being represented by curves, the rising barometer mostly coinciding with 

 the decreasing force of wind, and the falling barometer with its increase, so that a 

 rising and falling curve will with more or less fidelity represent the passage over a 

 station or a tract of country of the two compensating currents of Dove. It is not 

 the mere rise and fall of the barometer, as such, that constitutes an atmospheric 

 wave ; the barometric curve itself is doubtless the complex result of two or more 

 distinct variations of pressure connected with variations of wind as above. When 

 these are disentangled, the mind is able to grasp the onward march of the two par- 

 allel winds, accompanied by their respective pressures ; so that true waves of press- 

 ure really, I apprehend, sweep over a country ; and applying the wave nomenclature, 

 low pressures have been characterized as troughs and high pressures as crests. 



As illustrative of these remarks, I beg to exhibit on this occasion the most com- 

 plete instance of opposite pressures that has come under my notice ; it is the opposite 

 barometric curves at Alten and Lougan during the early part of November 1842 : the 

 curves will be found on page 39, ' Report,' 1848. I am indebted to Dr. Lee for the 

 observations furnishing the curve at Alten. 



