Sli'Temder 27, 1S94] 



NATURE 



525 



piece of a flageolet or of a small "whistle" ; and the 

 blowing across the end of a tube or a hole in the side 

 of a tube, to cause a key or a flute to sound. 



§4. Defalcation iIII.j is largely operative, and (II.) 

 but little, in many practical cases of most common 

 occurrence in the flow of water. It is probable that 

 much of the foam seen near the sides and in the wake 

 of a screw steamer going at a high speed through glassy- j 

 calm water, is due to "vacuum" behind edges and 

 roughnesses causing dissolved air to be extracted from 

 the water. A stiff circular disc of lo-inch diameter, 

 and no of an inch thick in its middle, shaped truly to 

 the figure of an oblate ellipsoid of revolution would 

 cause a vacuum ' to be formed all round its edge, if 

 moved at even so small a velocity as i foot per second 

 under water of any depth less than 63 feet ; if water 

 were inviscid : and at greater depths the motion would, 

 on the same supposition, be wholly continuous, with no 

 vacuum, and would be exactly in accordance with the 

 unique minimum energy solution.- 



While the velocity of the fluid across the equator is 

 637 feet per second, the velocity across each of the two 

 parallel circles whose radii are 4'2i 8 inches (the radius 

 of the equator being 5 inches) is only i foot per second. 



§ 5. The exceedingly rapid change of shape of the 

 fluid flowing across the equatorial zone between these 

 circles, with velocity at the surface augmenting from 

 I foot per second to 63 feet per second in advancing over 

 a distance of less than '85 of an inch of the surface from 

 one of the small circles to the equator, and diminishing 

 again from 63 to i from the equator to themher parallel, 

 in a small fraction of a second of time would, if the fluid 

 is water or any other real liquid, give rise, through vis- 

 cosity, to forces greatly diminishing the maximum velocity, 

 iind causing, through fluid pressure, the motion of the 

 ^vater to dift'er greatly from that of the minimum-energy 

 solution, nut only near the equator, or in its wake, or over 

 the rear side of the disk ; but over all the front side als(-, 

 though no doubt much more on tlie rear side and in the 

 wake, than on the front side, and in the fluid before it. 



The viscosity would also, at less depths than 63 feet, 

 have great effect in keeping down the maximum velucity ; 

 and it is possible that even at 10 or 20 feet a greater 

 velocity than i foot per second might be required to make 

 vacuum round the equator of our disc of 10 inches diameter 

 and the .j,-,l,<) of an inch radius of curvature which its 

 elliptic nieridianal section gives it. But it seems quite 

 certain that there must be much forming of vacuum, and 

 consequent extraction of air and rising of bubbles, to 

 the surface, from the somewhat sharp corners, and rough- 

 nesses, of iron, in the hull of an ordinary iron sailing 

 ship or steamer, going through the water at twelve knots 

 (that is, 20 ft. per second). Kelvin. 



{To be continued.) 



[Correction on previous short article, " Towards the 

 Efficiency of Sails, &c." In last line but two, for 277 

 substitute iir. In last line but one, delete 4, and for 8 

 substitute 32.] 



* Single word to denote space vacated by \vater._ 



- From the elementary hydrokinetics of the motiotlof an ellipsoid through 

 an inviscid incompressible fluid, originated by Green, who first gave the solu- 

 tion for the case of translational motion of the ellipsoid,_ we know that, if Q 

 denoting the angle between the axis of an oblate ellipsoid of revolution, of 

 which the equatorial and polar areas are <r, b, the velocity of the fluid flowing 

 over this point of the surface is 



\b-¥ 





if the velocity of the fluid at ?reat distances from the solid is V, and in parallel 

 lines, and the solid is held fixed in the fluid, with its axis parallel to these 



tines. Taking tt — ivod in this formula, we reduce it to -^°° V sin 9 ap- 



rr 

 proximately within i per cent. ; and taking: sin tf = t, and V= i foot per 

 second, wc (Ind ^^i'7 feet per sccord for the velocity across the equator. 

 Hence the praviiaiional head corresponding to the"* negative-pressure" is 

 (63'7- — I-) 64 "4, cr very approximately 63 lect, which proves the statement 

 in the text. 



SCIENCE, IN SCHOOL AND AFTER SCHOOL. 



TT is an unfortunate accident of the conditions under 

 ■•- which instruction in science has grown up, that in 

 speaking of science teaching two essentially dissimilar 

 things should be confused. This confusion has very seriously 

 affected — and still atiects — the develooment of method in 

 this country. It arises from the fact that, twenty or thirty 

 years ago at least, the ordinary schoolmaster was quite 

 without the knowledge necessary to teach science, and 

 that even when his scientific knowledge was a measurable 

 quantity, that ignorance of psychology which was and 

 which remains one of his most constant characteristics, 

 rendered him incapable of innovations upon the tradition 

 of mental training he cherished. Consequently what 

 knowledge people obtained of the growing body of 

 science came after the elementary stage of education was 

 over, when their minds and senses had already received 

 a considerable amount of cultivation and were, for good 

 or evil, definitely developed in a prescribed way. The 

 teaching given, therefore, did not aspire to be so much 

 educational as instructive ; it made the best of a bad job, 

 and without any belated attempts to alter the fundamental 

 intellectual mechanism, placed therein so much of the 

 new facts and views as the circumstances permitted. It 

 was addressed primarily to adolescence and to the adult, 

 its methods were by lecture, diagram and text-book, and 

 the written examination or a practical examination, 

 turning chiefly on the identification of specimens 

 or the interpretation of diagrams, was the adequate 

 measure of its value. .Such teaching can affect the taught 

 only through their opinions and knowledge ; it can dis- 

 cover scientific capacity, but it can neither develop nor 

 very largely increase it, because it conies too late in the 

 mental life. It is typically represented by the innumer- 

 able classes over which the Science and Art Department 

 presides. 



On the other hand, we have the science teaching that 

 is educational, that takes the pupil still undeveloped and 

 trains hand, eye, and mind together, enlarges the scope 

 of the observation, and stimulates the development of 

 the reasoning power. Such science teaching occurs at 

 present most abundantly in theoretical pedagogics. It 

 is, however, undoubtedly the proper science teaching for 

 the school, if science is to have a place in the school. For 

 it is universally conceded now.adays that the school is a 

 training place, that there the vessel is moulded rather than 

 filled, and that the only justification for the introduction 

 of science is its educational value. Equally indubitable 

 is it that it should be confined to school limits. .An 

 attempt to make the adult science teaching educational 

 in the same sense, would be— to complete the image— ex- 

 tremely like putting a well-baked — if imperfect— vessel 

 ' back upon the potter's wheel. 



Now, hitherto the chief influence of this confusion has 

 been to hamper truly educational science teaching in 

 schools. Those who had as adults studied science under 

 the Science and Art Department, or in University lecture 

 theatres, took their text-books and the methods under 

 which they had acquired their knowledge into the school, 

 where the conditions were altogether ditTerent. The 

 course of science lessons began as a lecture in which the 

 class listened to colourable imitations at second or third 

 hand of this or that eminent exponent of scientific theory. 

 The more discerning teachers after a time realised the 

 futility of requiring genuine lecture notes from such 

 immature minds, and supplied the deficiency /!^)' r/zV/ir/Zn^'' 

 a colourable imitation. They also provided copies on the 

 blackboard for such original sketches as were required, 

 and indeed went to very considerable pains to keep the 

 outward appearance of the lecture system intact. Ex- 

 aminers of schools — being selected without the slightest 

 reference to their capacity to examine— fell very readily 

 into this view, that school science-teaching was adult 



NO. I3CO. VOL. 50] 



