TRANSACTIONS OF SKCTION A. 



641 



38° W. 



P'lrinji the en-'iiin<r year witli imprdvod nppnrutiiH tlic nutbor lii^pcs to ninlio a 

 tiioiv (•(implftf scries of oljNi'vviitinn.s tiy wli'cli tlic variiilii)iif< in tin- velocity of (lie 

 iiir at (liil'crpnl hciirhts may lio disoiusst'd with relerencc lo the action oft lie several 

 I'iictors, (inio of day, season, tcmpcrntnre, liiiinidity, prt'ssurc, Sec. 'J'lie conclusion.^ 

 iit jtrcsent tentatively arrived at may be classified tliiis : 



'l. Tbe \<'liicity of tbe air invariably increuaes up to a lieif,'lit of ], 100 feet 

 above sea-b-vel. 



2. Tbe increase is rapid near tbe surfaco, and diminisbes sensibly at heights of 

 L'OO feet or more abovt^ tbe surface. 



t]. The velocity at any moderate bei^dit may be very neatly found from an 



empirical formula of the form = ( , ) wjThTaT where V, v, H,//, are tbe velocities 



iind beifrbts at tbe upper and lower altitudes, and a a constant to lo determined 

 by coni])!irisf)n of observation?. 



4. 'J'bat Kiippen's theory of tbe diurnal period in the velocity of tbe wind i>» 

 strong'ly supported by tbe l)elmviour of the kites. 



"). That there is an ascendinfj current in tbe front and a descending current in 

 the rear of travelling cumuli and cumulo-strati. 



It. On ilf ircmt Sini-tjhnr.t and Hahi in, connection n-i'th tlif Eruption of 

 Knil.vtua. Ihj Professor E. Douglas Auciiibald, M.A. 



!•'). On Whirlwinih anil WalfrsiKmls. 

 Bj Professor Jami:s Tiiom,son, LL.B., F.E.S. 



AVliirlwiiKls. whether on sea or on land, have their characters in great part 

 alike. I'or siiiiplieity it will be convenient to begin by taking up only tlie case of 

 whirhvinds on sea, as thus the necessity for alternative expressions to suit both 

 cases, that of sea and that of land, will be avoided. 



It may be aocejited asa fact suiliciently established both by dynanric theory and by 

 Itiirometric oljservations that, at tbe sea level, tbe prcssuri! of the air is less in the 

 neigbbourbood of the axis of whirl, than it is at ])!aces further out from tbe axis, 

 tlinugli within the region of tbe whirl. Tbe apocentrie force (centrifugal force) ol" 

 the rapidly revolving air resists tbe inward pulsive tendency of the greater outer 

 than inner pres.>ure. But close over tbe surface of the sea there exists necessarily 

 a lamina of air greatly deadened as to the whirling motion by iliiid friction, or 

 resistance, against tlie surfaco of the sea ; and, all tlio more so, because of that 

 surface being ruflted into waves, and often l)roken up into spray. This frictionally 

 deadened lamina exerts, because of i's diminished whirl speed, b^ss apocentrie 

 force tlian tbe rjuicker re\olving air above it, and so is incapable of resisting the 

 inward pulsive tendency of the greater outer than inner pressure already mentioned. 

 Hence, while rushing round in its whirl, the air of that lamina must also be 

 liowing in centreward. 



T e influx of air so amving at the central region cannot remain tl-.ere con- 

 tinually accumuhiting : it is not annihilated, and it certainly does not escape 

 downwards tlirougb the sea. There is no outlet for it except upwards, and, as a 

 rising central core, it departs from that place. This is one way of thinking out. 

 some of the conditions of the complex set of actions under contemplation, but 

 there is much more yet to be considered. 



Hitherto, in the present paper, nothing has been said as to the cause or mode 

 of origin of tbe diminished barometric pressure which, during tbe existence of the 

 wliirlwind, does actually exist in tbe central region. Often in writings on this 

 subject the notion has been set forth that tbe diminished pressure is caused by the 

 rapid gyratory motion of tbe whirlinp air; but were we to accept that view, we 

 would have still to ask: ' IIow does tbe remarkably rapid whirling motion receive 

 its own origin ? ' 'i"he reply must be that the view so oilered is erroneous, and that 

 in general a diminished pressure existing at some particular region is the cause 



1884, T T 



I 



