RESPECTING SOtTNI) AND LIGHT. 



535 



yield to the slightest impression of air. The 

 results are coiiectefl in Tables v and vi. 

 (Phite -2. Fig. 4. . 15.) In order to measure, 

 with greater certainty and precision, the ve- 

 locity of every part of the current, a second 

 cavity, furnished with a gage, was provided, 

 and pieces perforated with apertures of dif- 

 ferent sizes were adapted to its orifice : the 

 axis of the current was directed as accu- 

 rately as possible to the centres of these aper- 

 tures, and the resullsof the experiments, with 

 various pressures and distamjes, are inserted 

 in Tables vii, viii, and ix\ The velocity 

 of a stream being, both according to the 

 commonly received opinion, and to the expe- 

 riments already related, nearly in thesubdu- 

 plicate ratio of the pressure occasioning it, it 

 was inferred, that an equal pressure would be 

 required to stop its progress, and that the ve- 

 locity of the current, where it struck against 

 the aperture, must be in the subdupllcate ra- 

 tio of the pressine marked by the gage. 

 Having thus ascertained the velocity of the 

 stream at different distances from the aper- 

 ture, we must adopt, in order to infer from 

 it the magnitude of the stream, some suppo- 

 sition respecting the mode in which its mo- 

 tion is retarded, and the simplest hypothesis 

 appears to be, that the momentum of the 

 particles contained at first in a given small 

 length of the stream, together with that of 

 the particles of the surrounding air, which 

 they drag along, remains always constant, 

 so that the area of tlie transverse section may 

 be inversely as the square of the velocity ; 

 and the diameter inversely as the velocity it- 

 self ; the particles of the stream occupj'ing a 

 section as much wider as the velocity is 

 smaller, and carrying with them as many 

 more particles as will require a space still 

 larger in the same proportion. On this sup- 



position, the ordinates of a curve may 

 be taken reciprocally in the subdupll- 

 cate ratio of the pressure marked by the se- 

 cond gage to that indicated by the first, at 

 the various distances represei;ted by the ab- 

 scisses, and the solid, described by the revo- 

 lution of thi» curve round its ax's, will nearly 

 represent the magnitude of the current in 

 all its parts. (Plate a. Fig. l6. . 2(i.) As 

 the central particles must be supposed to 

 be less impeded in their motion than the 

 superficial ones, of course, the smaller the 

 aperture opposed to the centre of the current, 

 the greater the velocity ought to come out, 

 and the ordinate of the curve the smaller ; 

 but where the aperture was not greater ihan 

 that of the tube, the difference of the veloci- 

 ties at the same distance was scarcely per- 

 ceptible. When the aperture was larger than 

 that of the tube, if the distance was very small, 

 of coursethe average velocity came out much 

 smaller than that which was inferred from a 

 smaller aperture ; but, where the ordinate of 

 the internal curve became nearly equal to 

 this aperture, there was but little difference 

 between the velocities indicated with differ- 

 ent apertures. Indeed, in some cases, where 

 the diameter of the aperture was a little 

 greater than that of the stream striking on it, 

 it appeared to indicate a greater velocity than 

 a smaller aperture : this might have arisen 

 in some degree from the smaller aperture not 

 having been exactly in the centre of the 

 current ; but there is greater reason to sup- 

 pose, that it was occasioned by some resist- 

 ance derived from the air returning between 

 the sides of the aperture, and the current en- 

 tering it. Where this took place, the exter- 

 nal curves, which are so constructed as that 

 their ordinates are reciprocally in the subdu- 

 plicate ratio of the pressure observed in the 



