168 Prof. Magnus's Hydraulic Researches. 



through a tube which is narrowed towards the bottom. But in 

 such a case they are less distinct and sharply defined than when 

 the flow takes place through the thin wall. The same is true 

 on employing the magnetic hammer or the violoncello. 



Nature of the swellings. 



139. It is known that the swellings consist of separate isolated 

 masses of water. However regular their form may be, still the 

 separate masses may be recognized in them. If, for instance, a 

 piece of wire, or foil, or other light body be so held in the hand 

 that one of its ends penetrates a little way into the jet, a uni- 

 form pressure is felt when the substance is struck by the smooth 

 part of the stream. But if it be struck by a swelling, a strong 

 vibratory motion is felt. 



140. A flame which is brought into the neighbourhood of the 

 jet, burns calmly as long as it is held near the smooth part j 

 it is disturbed on being held near a swelling. The separate 

 masses of water which constitute the swelling, clearly communi- 

 cate their motion to the air, and this agitates the flame ; while 

 the smooth part of the stream, being continuous, scarcely moves 

 the air at all. 



141. In the investigations on the motions of liquids (Phil. 

 Mag. ser. 4. vol. i. p. 1), I have already mentioned {§ 3) that a 

 flame is not agitated on being brought near the continuous por- 

 tion of the stream. I have found this confirmed. But if, instead 

 of the flame of a candle, that of a Bunsen's gas-lamp be used, 

 the tap of which is only slightly opened, so that the gas escapes 

 only under a very feeble pressure, then the flame is drawn a 

 little down with the current when the latter passes through the 

 burning gas. It follows from this that even the continuous por- 

 tion of the stream draws the air somewhat with it ; but with so 

 small a force that, as soon as the gas leaves the burner under an 

 increased pressure, the flame is not carried along with the current. 



Observations by means of a rotating mirror. 



142. The nature of the jet can be examined with greater 

 accuracy if it be illuminated from a single point, and a mirror 

 be used for observing it in, of a few centimetres in breadth and 

 height, which revolves about a vertical axis. For the illumina- 

 tion, a simple lamp in a dark room is sufficient; it is better, 

 however, to employ a lamp in a lantern from which the light 

 is directed by means of lenses upon the portion of the jet to 

 be examined. In order to eff'ect the rotation of the mirror, I 

 generally employed a piece of clockwork, which M'as set up at a 

 distance of about a metre from the stream. If the mirror remains 

 in the same situation, only a small portion of the stream can be 



