Mr. Kemp's Description of a New Mercurial Air-Pump. 99 



descended below the level of the valve M, when it falls out of it- 

 self, and thus admits free access to the air from the receiver to re- 

 store the equilibrium : and as this takes place at every stroke of 

 the pump, when the air has even arrived at a very high state of 

 rarefaction, there is nothing whatever to prevent it from passing 

 along the tube H from the receiver to the exhausting vessel. 



The tube HH being terminated at the bottom of the exhausting 

 vessel, effectually prevents any return of air through the valve into 

 the receiver ; and as the cup wherein the valve G is placed, always 

 contains a portion of mercury, no air can return into the exhaust- 

 ing vessel without first forcing this through, which never can take 

 place while the pump is in working order. 



By an air-pump of this construction, we can produce a much 

 greater degree of rarefaction than when pistons and valves are 

 used ; for here the pistons do not directly produce the vacuum, 

 this being dependent on the ascent and descent of the mercury 

 into the exhausting vessels, where a Torricellian vacuum is pro- 

 duced at each stroke of the pump. 



ART. IV. Description of a New Species of British Fish. By 

 Captain Thomas Brown, F.L.S. &c.* 



Platessa carnaria. 

 The Flesh-coloured Fluke. Plate III. 



Head very large, being a third of the length of the fish, to the 

 insertion of the caudal fin ; eyes on the right side ; irides orange, 

 the nictiating membrane green ; behind the eyes, the head is cover- 

 ed with prickly scales. Body ovate, the upper side covered with 

 very small, smooth, concave adherent scales ; flesh-coloured, with 

 irregular deep rose-coloured distant spots. A fillet of small sharp 

 fasciculated spines, run longitudinally at the junction of the dorsal 

 and anal fins. Under side smooth, convex, and silvery white. 



Length 5 4 inches, breadth 3i inches. 



This species was procured, along with a quantity of the Platessa 

 fesus, or common iluke, from Prestonpans. At first I considered 

 it as merely a variety of that species, but it diflFers, in being much 

 shorter in proportion to its breadth ; in being more convex on both 

 sides ; in the head being greatly larger, measuring 1 and |ths of an 

 inch, or exactly a third of the whole fish, exclusive of its tail fin : 

 whereas the body of the flesus is half a head longer. The dorsal 

 fin consists of sixty, and the ventral fin of forty-one rays ; while 

 the former of ihejlesus has only fifty-five, and the latter forty rays, 

 and they are considerably different in their external shape. This 

 of itself is a sufficient specific distinction. 



• Read before the Royal Physical Society April 7. 1830. 



