136 DR. WOLLASTON ON A DIFFERENTIAL BAROMETER. 



prevalence of a sea-breeze, he had observed the barometer to be in general per- 

 ceptibly higher than when a land-wind prevails ; and he endeavours ingeniously 

 to account for this appearance by the accumulation of air which takes place 

 in front of any obstacle opposed to the air's motion, and which, therefore, 

 occasions in it a greater barometric density. 



It was on this principle that a species of wind-gauge was constructed by Dr. 

 LiND, consisting of an inverted siphon, having the extremities of its two legs 

 bent horizontally and in opposite directions. When the siphon is partially 

 filled with water, and one of its horizontal extremities is exposed to a current 

 of air, its pressure occasions the water on this side to descend, until its force is 

 counteracted by the greater height of water in the opposite leg, the difference 

 of the two columns being the measure of the force of the wind. 



If a lighter fluid than water were employed in the construction of Dr. Lind's 

 instrument, it would be rendered proportionally more sensible ; but to such 

 means of improvement there is a natural limit, since the scale could not, by 

 means of any known fluid, be increased in a greater ratio than that of 4 to 5. 

 Whereas by means of the instrument which is here proposed, the range of the 

 index may be increased in any desired proportion, so as to measure the force 

 of the gentlest flow of air. 



TO nuq 



