432 M. J. Plateau on Jets of Liquid under the 
that of the instrument. In order that the vibrations of the in- 
strument may be capable of producing this effeet, however, it is 
evidently necessary that they should have a sufficient intensity 
when compared with that of the vibrations of the note natural 
to the jet, since the tendency of the latter is in favour of the 
normal action of the forces of figure. 
We shall understand the phenomenon still better by regard- 
ing it from a slightly different point of view. The vibrations, it 
will be remembered (§ 5), tend of themselves to produce contrac- 
tions and expansions upon the jet ; now if these expansions and 
contractions differ but little in length from those which the forces 
of figure tend on their part to produce, and if, moreover, the 
action of the vibrations is sufficiently intense to predominate 
over that of the forces of figure, the resulting system of contrac- 
tions and expansions ought to be that which depends upon the 
vibrations, and afterwards the transformation, thus modified from 
its origin, ought to complete itself according to the new manner. 
But this condition of the jet is a forced one, because the 
natural mode of transformation is altered. Consequently, if from 
any cause the regular succession or transmission of the vibrations 
becomes disturbed, the forces of figure ought immediately to pre- 
ponderate again, and the contractions and expansions to reassume 
the length which corresponds to the free action of these forces. 
Thus is explained without difficulty the peculiarity in the expe- 
riment of No. 10 of paragraph 3, according to which a slight 
shock imparted to the apparatus, or a change in the position of 
the body upon which the jet falls, is often sufficient to cause the 
note of the jet to return suddenly to its natural pitch. 
We have assumed that in this experiment the note of the jet 
puts itself in unison with that of the body receiving the shock, 
in conformity with the principle advanced at the commencement 
of this paragraph. Nevertheless, as may be concluded from the 
enunciation in the No. in question, Savart does not express 
himself on this point in precise terms; he says merely that the 
note of the body receiving the shock modifies that of the jet, 
whose pitch it changes; but other experiments which we shall 
soon have to discuss permit us to attribute to these words the 
meaning we have above given them. 
§ 16. Lastly, No. 10 of paragraph 3 also informs us, that 
when the interval between the two notes is very small, both of 
-them may be heard periodically, or even simultaneously. Let 
us endeayour to explain these facts also. 
Let us suppose that the note natural to the jet has a some- 
what lower pitch than that of the body receiving the shock. In 
the case of exact unison, the number of impulses proceeding from 
the detached masses in a given time would be equal to half the 
