VOL, XXXVIII.] PHILOSOPHICAL TRANSACTIONS. 5g3 



between the east and north points of the compass If therefore a special wind 

 should spring up and blow in a contrary direction, it will produce tremulous 

 vibrations, and consequently dilatattons of the air; then equal bulks of the di- 

 lated air dilated, will have a less quantity of matter than before; therefore the 

 gravity of the air will be lessened, and by consequence the quicksilver in the 

 weather-glass will fall. And this decrease of gravity in the air, and of the 

 height of the mercury in the baroscope, will be proportional to the intensity of 

 the force of the wind, and the degree of opposition of its direction to that of 

 the flux of the atmosphere, conjunctly. 



This, says Mr. Gersten, is the reason why the mercury falls when southerly 

 or westerly winds blow, and why the quicksilver sinks so very low when these 

 winds blow stormy. On the contrary, since the effect ceases when the cause 

 is removed, the height of the mercury will be greater, the fewer special winds 

 there are blowing in a contrary direction. So that the gentle winds that blow 

 from the points of the compass which lie between the north and the east are, 

 he believes, nothing but the natural and universal motion, current, or flux of 

 the atmosphere, impeded by or meeting with very few special fluxes. 



The last chapter is employed in accounting for the various changes of the 

 weather connected with, or consequent on the rise and fall of the mercury in 

 the weather glass. The ingenious author, beginning with the origin and man- 

 ner of forming vapours, undertakes to settle and confirm, on solid principles, 

 that which the learned and sagacious Dr. Halley had long ago communicated 

 to the learned world, on this argument. 



The Design of the Disser union annexed, is to inquire into the Nature of Dew, 

 explain its Origin and Kinds. — All dews, according to our author's philosophy, 

 owe their origin either to vegetables or terrestrial ascending exhalations. Such 

 as derive their origin from vegetables, he takes to be only exudations of their 

 leaves, &c, congealed by the air. 



In treating of that kind of dew which is a secretion or exudation of a juice 

 in vegetables, he observes, that some plants furnish the spectator with a very 

 entertaining sight, the little drops of dew being disposed after a very regular, 

 not fortuitous m.inner, on the surfaces or edges of their leaves. To determine 

 whether this beautiful disposition of the dewy particles is owing to a descent 

 from the chilled air over the plant, or a secretion made from the juices of the 

 plant itself, he covered several with glasses, or earthen vessels, having their 

 mouths downwards; and yet the next day plenty of this kind of dew appeared 

 in its usual regular form. 



As to the next species, or common dew, he produces so many, and so dif- 

 ferently made experiments, against the common opinion of its descent, that if 



VOL. VII. 4 G 



