J49 M. R. GlausiiwVJ^^fy /ol^i-oif. (Thomson's Note 



oritiioe to be much iess than in the orifice itself, and which there- 

 fore destroys the greater part of the said effect^ that om' views 

 differ from each othef. > mh }\ )Ci;;/:';H <fiir*i- xit >u :!-infn>M.;ii; 



^Mr. Thomson explains this fact by i-feferring it to ^^fthe fric^ 

 tion of the steam as it rushes through the orifice*/' In order 

 rightly to estimate the ])ossible influence exerted by this friction, 

 its action must in the first place be more clearly characterized. 

 This action does not consist in the loss of a velocity which the 

 steam had already attained, but in the circumstance that the 

 steam from the commencement never attains the velocity which 

 it would have done had friction been entirely absent. That ve- 

 locity, on the contrary, which it possesses in the orifice, and 

 which it loses further on, cannot be lost by friction. It is in no 

 way difficult to demonstrate the actual ground of this loss. 



For this purpose we will retain the assumption, which, for the 

 sake of simplifying the matter, I have introduced in my investi- 

 gation, that the orifice is fui-nished with a widening neck (see 

 fig. vol. i. p. 403 1), and that up to the point where we suppose 

 the loss of velocity to have already taken place, th^ steam has 

 remained unmixed with atmospheric air. 



What takes place in the neck may be represented somewhat 

 more clearly by means of an apparatus of the shape shown in 

 the figure. AB is a narrow tube f^> > > ^ 



fastened by means of a closely-fitting K 



cork in the w ider tube CDEF, which ■ ■ ii i , mn, ! . ^- 



latter is furnished with a siphon- ^ ^ I II I f ^ .h-cMiol 



shaped tube partially filled with fluid, 

 by means of which the pressure 

 within may be observed. If we blow 

 through the narrow tube from A ^ 



towards B, so that the current of air can expand itself in the 

 wide tube before it reaches the open atmosphere, it is well known' 

 that the fluid immediately rises in the leg HG and sinks in HK. 

 In the vicinity of B a smaller pressure exists than that of the 

 atmosphere which acts at K and EF. It is this difference of 

 pressure which destroys so much of the original velocity of the 

 current of air on its way from B to EF, that the same quantity 

 of air which passed the orifice B during the imit of time can 

 during the unit of time fill the cross section EF. Let it be 

 imagined that a cmTcnt of steam from a high-pressure boiler 

 passes thi'ough the tube AB instead of the current of air, we 

 have then in the interior of the wide tube a retarding force, 



* Phil. Mag., vol. xxxvii. p. 388. 



t In this fiffiire, as already stated in the eiTata to Poggendoi*ff' s Annalen, 

 the surface GIII must be moved somewhat further from the orifice towards 

 the centre of the vessel. 



