ON THE rKESSURE OF WIND CrON .V KIXED PLANE SURFACE. 481 



From this easily reaiembered formula the following table of pressures is 

 constructed : — 



In general only these, the maximum pressures, are required ; but 

 sometimes, as in the case of the inclined sail of a windmill or ship, or 

 the roof of a building, the diminished pressure upon a surface placed 

 obliquely to the effective current is needed: -we have then 



F 



_ /v sin dV' 



in which v = the absolute velocity with v?hich the air strikes the receding 

 plane ; and 6 = the internal angle made by the obliquely placed surface 

 and the direction of the impinging wind. 



With regard to the phenomenon called ' a gust of wind,' nothing is 

 known either as to its cause, or as to its exceptional but almost momen- 

 tary velocity, or as to the extent of the area over which it temporarily 

 operates ; but it is, notwithstanding, certain that a wind-pressure of even 

 40 lbs. on the square foot is unknown in these islands, because, as may be 

 readily shown, this intensity of pressure would have sufficed to overthrow 

 most of the long-existing factory chimneys, to overset post windmills, 

 and to scatter the greater number of the slighter built domestic and other 

 structures which have nevertheless ' weathered many a storm,' and still 

 remain intact. 



It remains to make a passing allusion to whirlwinds, tornadoes, and 

 waterspouts, all the results of spiral motions apparently produced in 

 some obscure manner by electrical action. These phenomena are very 

 rarely observed to occur on an important scale in these kingdoms. The- 

 powerful forces concerned in or generated by these erratic movement3 

 have never been measured, and consequently cannot be formulated ; but 

 it may be observed that, were they known, they could not be introduced 

 with propriety into calculations of the strength of structures intended to 

 have a commercial value, because of the extreme improbability of any 

 particular structure falling within the range of their destructive effects. 

 They are, in fact, within the legal category of ' Acta Dei.' 



The conclusion of the author of this paper, therefore, ia that for 

 structural calculations a maximum wind-pressure of 40 lbs. per square 

 foot may be vei-y safely adopted, notwithstanding some reported anemo- 

 metrical observations to the contrary. 



With regard to these observations the author remarks that the 



1881. I I 



