VOL. XLVI.] PHILOSOPHICAL TRANSACTIONS. 651 



merit, which was to make the incision or enlargement ; viz. the 3 instruments 

 in one: and this instrument was attended with this advantage, that it could 

 serve for men as well as women. One hand is sufficient to perform the operation 

 with this instrument. 



A is a ring, for passing the middle finger of the right hand, which grasps the 

 handle g. b another ring, which is slid by the fore-finger towards d, to push the 

 blade ab, out of the groove ef. The figure represents the instrument in the 

 state in which it is at the very instant when the incision is made into the neck 

 of the bladder. The same fore-finger draws the piece b back towards a, when 

 the operator intends to make the blades return into the groove f ; where they 

 lie hid, while he introduces the instrument from d, as far as Ff into the urethra. 

 The groove pe is closed or covered from d to e, in order to secure the pieces 

 a, b, c, d, e, in situation. The pins, which bind the hinges a, b, c, must not 

 be in the centre of the pieces, but as they are expressed in the figure , where b 

 is pretty near the outside edge, and the other two, a, e, on the opposite side ; to 

 the end that, when the ring b is thrust forward, the hinge b may bend, and 

 issue out of the groove p, by making the angle abc. 



On the Agreement of Barometers with the Changes of the ff'eather. By Samuel 

 Christian Hollman. From the Latin, N° 492, p. 101. 



Mr. H. observes, that it has hitherto been deemed a matter of great difficulty 

 to explain the true cause of the alteration in the height of the quicksilver in the 

 baromet^, and the manner of its agreement with the subsequent changes of the 

 weather. He then remarks on the hypothesis of Leibnitz on these points, which 

 had for some time prevailed, viz. that by the drops of rain falling through the 

 atmosphere, this is deprived of so much of its weight; showing that this notion 

 is not authorised by either theory or practice, and that it had been shown by 

 Desagviliers (Philos. Trans. N° 351 ) to be quite contrary to the laws of hydrosta- 

 tics ; by which it is found that solids, descending in fluids, add their weight to 

 the ambient fluid. 



Mr. H. remarks on the popular illusion, that the air is thought to be heavier 

 when it becomes turbid and rep'ete with thick vapours, and light when the air 

 is clear and bright ; which are contrary to fact, and the constant experience of 

 the barometer. He argues, that the air being, by any means, rendered lighter, 

 this levity is the cause of the formation of the contained vapours into a cloudy 

 and thick appearance in the atmosphere, and the consequent descent of rain. 

 But, besides that these phenomena are not constant concomitants ; that a thick 

 atmosphere does not always attend the descending barometer, nor rain follow it ; 

 the cause of the atmosphere's becoming lighter is still unexplained ; though Mr. 

 H. thinks it may be produced by various means ; such as the alternate heating 



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