pifton. 



308 AIR *UMf.l 



traction, and might be detrimental in fome experiments, the 

 diameter of the dud A is made very (mail, and another barrel, 

 having fimilar piftons and duels, is added, with its rack placed 

 above the pinion wheel, while the other is placed below it. 

 The advantage of a pump of this kind being conftructed with 

 two barrels arifes from the contrary motions of their piftons ; 

 for while one augments the capacity of the receiver by moving 

 forward, the other equally diminifhes it by moving backward. 

 An equilibrium is thus maintained that prevents any ofcillatory 

 motion in the mercury of the gage, which might arife from the 

 operation of a tingle barrel. 

 The refiftance The additional refinance to be overcome in working this 

 nifton fprmS P um P> above what is to be met with in other pumps, happens 

 only at the fma!J interval while the fpring pifton is paffing from 

 its natural fituation to the du6t B. This need not be more than 

 about four times greater than that which is requifite to over- 

 come tbefri&ion of the pifton P, and will be eafily provided 

 or by increafing the proportion between the diameter of the 

 pinion wheel and the fweep of the handle. 



ELIZUR WRIGHT, C.A.S. 



Canaan (Connecticut in America,) 

 March 12, 1805. 



II. 



Concerning the Stale in which the true Sap of Trees is depofded 

 during Winter, By Thomas Andrew Knight, Efq* 



(Concluded from Page 240) 



bulbous andtu- VV E have much more decifive evidence that bulbous and 



berous roots tuberous rooted plants contain the matter within themfelves 



ter* that forms " wn * cn fubfequently compofes their leaves; for we fee them 



.leaves, vegetate even in dry rooms, on the approach of fpring; and 



many bulbous rooted plants produce their leaves and flowers 



with nearly the fame vigour by the application of water only, 



as they do when growing in the beft mould. But the water in 



this cafe, provided that it be perfectly pure, probably affords 



little or no food to the plant, and ads only by diflblving the 



* Phil. Tranf. of 1805, p. 91. 



matter 



