170 Professor C. Piazzi Smyth's Meteorological 



always caused an alteration in the position of the vane at the 

 mast head. While, so far as the effect on the strength of the 

 wind is concerned, who in the navy has not heard of crack 

 frigates sailing straight before a violent wind, and acquiring 

 so nearly its velocity, that though the waves are rolling high, 

 and would be deemed stormy if steered against, yet a naked 

 lighted candle can be safely held on the quarter-deck, when 

 the bystanders, acted on by the difference only between the 

 velocities of the wind and the vessel, feel but a gentle air ? 



That common observation should have detected the sensible 

 effect of a cause, even lately declared by a high scientific 

 name to be insensible in more accurate instrumental deter- 

 minations, seems to be mainly attributable to the species of 

 instrument employed, viz., Lind's anemometer, being not of 

 the proper principle ; and one has only to look at the great 

 extent of the momentary fluctuations of the fluid therein, on 

 its extremely small scale too, to extend indulgence to those 

 who despaired of being able to read off the mean quantity to 

 any degree of accuracy worthy of minutely calculated cor- 

 rections. 



But although by enlarging and otherwise altering the in- 

 strument, a more certain and stable indication may be pro- 

 duced, yet all classes of anemometers giving, like this one, 

 pressures instead of velocities^ may be considered as fairly 

 knocked on the head, since Dr Robinson has shewn (in the 

 able description of his Armagh anemometers recently pub- 

 lished in the Transactions of the Royal Irish Academy), that 

 while the latter elements, rather than the former, are re- 

 quired in all meteorological questions, that they are also 

 more uniformly given, variations of velocity bearing a com- 

 paratively small ratio to those of pressure. 



The direction of the wind has always been much better 

 observed at sea than the force, i. e., velocity ; for the former, 

 there has always been an instrument used, viz., the vane and 

 compass ; but for the latter, there has been nothing but the 

 ordinary sensations and fancies of men ; hence it is much 

 more safe to found meteorological theories on the former data 

 rather than the latter: and in this respect the case is some- 

 what similar to the computation of the orbits of double stars, 



