When the sphere is _not_ well _dried_ and _polished_.]

[Ill.u.s.tration: SERIES VIII.

_The Splash of a Solid Sphere_--(continued.)

When the sphere is _rough_ or _wet_.]

[Ill.u.s.tration: SERIES IX.

_The Splash of a Solid Sphere_--(continued.)

When the sphere is rough or wet, and falls above 5 feet.]

Figs. 6 and 7 show the crater falling in, but this did not always happen, for the walls often closed over the hollow exactly as in Figs. 4 and 5 of Series IV. Meanwhile the long and nearly cylindrical portion below breaks up into bubbles which rise quickly to the surface.

By increasing the fall to 5 feet we obtain the figures of Series IX. The tube of Fig. 1 corresponds to the dome of Series IV. and V., and is not only elevated to a surprising height, but is also in the act of cleaving (the outline being approximately that of the unduloid of M. Plateau).

Figs. 2 and 3 show the bubble formed by the closing up of this tube, weighed down in the centre as in Figs. 5 and 6 of Series V. Similar results were obtained with other liquids, such as petroleum and alcohol.

It is easy to show in a very striking manner the paramount influence of the condition of the solid surface. I have here a number of similar marbles; this set has been well polished by rubbing with wash leather. I drop them one by one through a s.p.a.ce of about 1 foot into this deep, wide, cylindrical gla.s.s vessel, lighted up by a lamp placed behind it.

You see each marble enters noiselessly and with hardly a visible trace of splash. Now I pick them out and drop them in again (or to save trouble, I drop in the place of these other wet ones), everything is changed. You see how the air is carried to the very bottom of the vessel, and you hear the "phloisbos" of the bubbles as they rise to the surface and burst. These dry but rough marbles behave in much the same way.

Such are the main features of the Natural History of Splashes, as I made it out between thirteen and eighteen years ago. Before pa.s.sing on to the photographs that I have since obtained, I desire to add a few words of comment. I have not till now alluded to any imperfections in the timing apparatus. But no apparatus of the kind can be absolutely perfect, and, as a matter of fact, when everything is adjusted so as to display a particular stage, it will happen that in a succession of observations there is a certain variation in what is seen. Thus the configuration viewed may be said to oscillate slightly about the mean for which the apparatus is adjusted. Now this is due both to small imperfections in the timing apparatus and to the fact that the splashes themselves do actually vary within certain limits. The reasons are not very far to seek. In the first place the rate of demagnetization of the electro-magnets varies slightly, being partly dependent on the varying resistance of the contacts of crossed wires, partly on the temperature of the magnet, which is affected by the length of time for which the current has been running. But a much more important reason is the variation of the slight adhesion of the drop to the smoked watch-gla.s.s that has supported it, and consequently of the oscillations to which, as we shall see, the drop is subjected as it descends. Thus the drop will sometimes strike the surface in a flattened form, at others in an elongated form, and there will be a difference, not only in the time of impact, but in the nature of the ensuing splash; consequently some judgment is required in selecting a consecutive series of drawings. The only way is to make a considerable number of drawings of each stage, and then to pick out a consecutive series. Now, whenever judgment has to be used, there is room for error of judgment, and moreover, it is impossible to put together the drawings so as to tell a consecutive story, without being guided by some theory, such as I have already sketched, as to the nature of the motion and the conditions that govern it. You will therefore be good enough to remember that this chronicle of the events of a tenth of a second is not a mechanical record but is presented by a fallible human historian, whose account, like that of any other contemporary observer, will be none the worse for independent confirmation. That confirmation is fortunately obtainable. In an attempt made eighteen years ago to photograph the splash of a drop of mercury, I was unable to procure plates sufficiently sensitive to respond to the very short exposures that were required, and consequently abandoned the endeavour. But in recent years plates of exquisite sensitiveness have been produced, and such photographs as those taken by Mr. Boys of a flying rifle bullet have shown that difficulties on the score of sensitiveness have been practically overcome. Within the last few weeks, with the valuable a.s.sistance of my colleague at Devonport, Mr. R.S.

Cole, I have succeeded in obtaining photographs of various splashes.

Following Prof. Boys' suggestion, we employed Thomas's cyclist plates, or occasionally the less sensitive "extra-rapid" plates of the same makers, and as a developer, Eikonogen solution of triple strength, in which the plates were kept for about 40 minutes, the development being conducted in complete darkness.

A few preliminary trials with the self-induction spark produced at the surface of mercury by the apparatus that you have seen at work, showed that the illumination, though ample for direct vision, was not sufficient for photography. When the current strength was increased, so as to make the illumination bright enough for the camera, then the spark became of too great duration, for it lasted for between 4 and 5 thousandths of a second, within which time there was very perceptible motion of the drop and consequent blurring. It was therefore necessary to modify the apparatus so as to employ a Leyden-jar spark whose duration was probably less than 10-millionths of a second. A very slight change in the apparatus rendered it suitable for the new conditions, but time does not permit me to describe the arrangements in detail. It is, however, less necessary to do so as the method is in all essentials the same as that described in this room two years ago by Lord Rayleigh in connection with the photography of a breaking soap-film.[4]

I therefore pa.s.s at once to the photographs themselves.

The first two series (X. and XI.) may be described as shadow photographs; they were obtained by allowing a drop of mercury to fall on to the naked photographic plate itself, the illuminating spark being produced vertically above it, and they give only a horizontal section of the drop in various stages, revealing the form of the outline of the part in contact with the plate, but of course telling nothing about the shape of the parts above. The first series corresponds to a mercury splash very similar to that first described, and the second to the splash of a larger drop such as was not described. In each series, the tearing of the thin central film to which allusion was made is well ill.u.s.trated. I think the first comment that any one would make is that the photographs, while they bear out the drawings in many details, show greater irregularity than the drawings would have led one to expect. On this point I shall presently have something to say.

SERIES X.

(1) _Instantaneous Shadow Photographs (life size) of the Splash of a Drop of Mercury falling 8 cm. on to the Photographic Plate._

[Ill.u.s.tration: 1

Actual size of the Drop, 483 mm.]

[Ill.u.s.tration: 2

[Tau] = 0]

[Ill.u.s.tration: 3]

[Ill.u.s.tration: 4]

[Ill.u.s.tration: 5]

[Ill.u.s.tration: 6]

[Ill.u.s.tration: 7

[Tau] = 048 sec.]

SERIES XI.

(2) _Instantaneous Shadow Photographs (life size) of the Splash of a Drop of Mercury falling 15 cm. on to Gla.s.s._

[Ill.u.s.tration: 1

Actual size, 483 mm. in diameter.]

[Ill.u.s.tration: 2

[Tau] = 0 sec.]

[Ill.u.s.tration: 3]

[Ill.u.s.tration: 4

4A

[Tau] = 0032 sec.]

[Ill.u.s.tration: 5

[Tau] = 0063 sec.]

[Ill.u.s.tration: 5A

[Tau] = 0094 sec.]

[Ill.u.s.tration: 6

[Tau] = 0134 sec.]

Comparing the first set of drawings (pp. 20-24) with the photographs of Series X., it will be seen that

Photograph 2 corresponds to drawing 4 or 5 " 3 " " 9 " 4 " " 18 " 6 " " 20 " 7 " " 24

but the irregularity of the last photograph almost masks the resemblance.

SERIES XII.

_Engravings from Instantaneous Photographs (16/17 of the real size) of the Splash of a Drop of Mercury, 483 mm. in diameter, falling 89 cm.