174 farmers' and mechanics' journal. 



pressed on the coin or medal by means of this force, which was 

 first accumulated by the fly, and afterwards augmented by the in- 

 tervention of two mechanical powers. 



" Notwithstanding the great advantages of fly-wheels, both as 

 regulators of machines and collectors of power, their utility wholly 

 depends upon the position which is assigned them, relative to the 

 impelled and working points of the engine. For this purpose, no 

 particular rules can be laid down, as their position depends alto- 

 gether on the nature of the machinery. We may observe, how- 

 ever, in general, that when fly-wheels are employed to regulate 

 machinery, they should be near the impelling power ; and whea 

 used to accumulate force in the working point, they should not be 

 far distant from it. In hand-mills for grinding corn, the fly is, for 

 the most part, very injudiciously fixed on the axis to which the 

 winch is attached ; whereas it should always be fastened to the 

 upper millstone, so as to revolve with the same rapidity. In the 

 first position, indeed, it must equalize the varying efforts of the 

 power which moves the winch ; but when it is attached to the 

 turning millstone, it not only does this, but contributes very eflfect- 

 ually to the grinding of the corn. 



" Dr. Desaguliers mentions an instance of a blundering engineer, 

 who applied a fly-wheel to the slowest mover of the machine, in- 

 stead of the swiftest. The machine was driven by four men, and 

 when the fly was taken away, one man was sufficiently able to 

 work it. The error of the workman arose from his conceiving, 

 like many others, that the fly added power to the machine ; but we 

 presume that Dr. Desaguliers himself has been accessory to this 

 general misconception of its nature, by denominating it a mechani- 

 cal power. By the interposition of a fly, however, as the Doctor 

 well knew, we gain no mechanical force ; the impelling power, on 

 the contrary, is wasted, and the fly itself even loses some of the 

 force which it receives by the resistance of the air." 



\^London Mcch. Mag. 



On the difficult^/ of separating flat Disks by a current of air, or of 

 steam, when their surfaces are in contact, or nearly so. By Asa 

 Spencer, Mechanician. 



Sir, — Agreeably to my promise, 1 offer for inseition in your 

 Journal, my theory respecting the adhesion of disks, which 1 shall 

 be pleased to see published, should it meet your views. 



Yours, &c. Asa Spencer. 



Tn the Journal of the Franklin Institute of last year, was pub- 

 lished a communication by M. Clement of the following purport : 



A flat valve being placed over an orifice in a steam chest, and 

 covered a considerable space around the orifice, when the steam 

 was let on, the valve, instead of rising, as was expected, kepi its 

 place, and required a considerable additional force to lift it. 



To this phenomenon I offered the following explanation to some 

 of my scientific friends, who thought it satisfactory : 



