-- '^^mthe'EffeS of Flvid Fricti(m},^^c.'^ J^ T41 



which is evidfefltiy independent of the small friction whi^h'^&iifS 

 takes placeJ^i^^ •^■>"''^i'» ^''^^^ '^f"^' '^" ^"'^''f '«'>^'''-''r5 "iii' H/mi«oL a-joi 



Inasmuch as the steam between B and E^ taes'the "^rfeMi* 

 part of the vis viva which it possessed at B, a quantity of heat 

 equal to that formerly expended in the production of this portion 

 of the vis viva must be again produced ; this portion of the vis 

 viva does not therefore at all enter into the calculation of the 

 entire quantity of heat consumed up to EF ; so that, without 

 overlooking it, I might leave it altogether unmentioned. ■ -'^"^^^ 



We must now consider those cases where the orifice is unfil'?- 

 nished with either a widening neck or a tube such as we have 

 described,, but where the orifice opens immediately into the atmo- 

 sphere. And here we will choose an extreme case; that is, 

 where the orifice is at a tube-end which opens into the free 

 atmosphere, and the issuing steam has not only the air in front 

 of it, but is encompassed by it on all sides. ^ i^jn (iikjmfe tu \iAiiH 



The current immediately after its exit sets a'ktge ^^ss 6f to 

 in motion ; not only the air before the opening, but also that 

 behind the same. For inasmuch as a portion of the air before 

 the opening is carried forward with the steam, a decrease of den- 

 sity takes place in the immediate neighbourhood of the orifice, 

 and thus the air behind will be drawn forwards. This deport- 

 nient is not only deducible on theoretic principles, but for the 

 analogous case of a current of w^ater streaming into a reservoir of 

 the same fluid, it is proved experimentally by Venturi*, Von 

 Feilitzscht, and recently by Magnus J, so that I may assume it 

 as known. By this communication of motion the velocity must 

 naturally diminish in proportion as the mass moved increases ; 

 inasmuch as the quantity of motion, that is, the product of mass 

 and velocity, cannot become greater. But if this product remains 

 constant, the product of the mass and the square of the velocity, 

 that is, the vis viva, must become smaller ; and we must assume 

 that the vis viva which thus disappears again makes its appeatf-, 

 ance as heat. • ' " 



All other possible cases with respect to the position of the 

 orifice, for example, that generally considered where the orifice 

 is situate in a wide plate, so that the steam after its issue has 

 the air all around it in front and the plate behind, lie between' 

 those two already considered. These form, so to say, the tWo 

 limits ; and by comparison with these we can alw^ays obtain an 

 approximate idea of the phsenomena under consideration. If it 

 be even granted that in all other cases the vis viva lost by the 



* Recherches Experimentales sur le Principe de la Communication hatf- 

 rale du Mouvcment dans les Fluides. ^,.^ ',. j^ 



t Pogg. ^riw., vol. Ixiii. p. 216. . . V*rii>orft 



:|: Pogg. Ann,, vol. Ixxx. p. 1 ; and Phil. Mag. 4th ser. vol.'il pi 1'. * ' 



