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" fwift a current, fo that the air muft be attenuated where the 

 " faid winds continue to blow." — " Add that the horizontal 

 " motion, being fo quick, may take off fome part of the per- 

 " pendicular preffure." This laft reafon feemed to acquire fome 

 confirmation from an experiment made by Mr. Hawkfby ; for 

 having paffed a ftream of air through a box in which the lower 

 fhank of a barometer was inferted, this ingenious gentleman 

 obferved the ^ to fall while the current paffed through the box, 

 as alfo in another barometer which communicated with the box, 

 but over which the current of air did not flow. 



Yet if all this were allowed, ftill the phaenomenon in queftion 

 would remain unexplained ; for not only during the florm, but 

 feveral hours, if not days before it, the mercury defcends confider- 

 ably, as Dodor Halley himfelf and all thofe who recommend the 

 marine barometer atteft, otherwife this inftrument would be ufe- 

 lefs. Mr. Cafwell fays that two days before the great ftorm of 

 January, 1734-5, the 5 fell 7^ of an inch below 28 inches*. 

 But even if the. fall were barely concomitant with the ftorm, 

 Dodor Halley's reafons would not prove their connexion. That 

 a body fliould move through air with fuch velocity as to leave 

 a vacuum behind it, there is a neceflity that it fhould move at 

 the rate of 1 1 or 1200 feet per fccond, as Mr. Robins has fliewn ; 

 now the fwifteft wind moves only at the rate of 92 or 93 feet 

 per fecond, as appears by the obfervations of Mr. Brice and 

 many others f. 



* VIII. Phi). Tranf. Abr. p. 458. 

 t Phil. Tranf. 1766, p. l66. 



The 



