NATURE 



\_y21ne 10, 1880 



tration of a pipe was used with the endeavour to aid the con- 

 ceptions in some respects, rather than for rigid accuracy 0' 

 comparison. The idea of the exterior fluid being at rest was 

 subsequently guarded against by stating that it had " important 

 functions " to perform. In regard to the fact of only mentioning 

 " friction " as an element of resistance in a totally immersed 

 body, I wished rather to convey the general idea that if no 

 enci-gy "ere given to the molecules of the surrounding liquid at 

 the passage of the immersed body, there would be no " resist- 

 ance. " The object of the article was, however, not so much to 

 lay stress on these points as to notice certain, perhaps less 

 appreciated {a priori), aspects of the problem. 



S. ToLVER Preston 



Songs of Birds 



Your correspondent " A. N." {antea, p. 97) does not seem to 

 be aware that the best observers are nowadays agreed m believing 

 that the hen cuckoo does not sing. Hence his suggestion in 

 regard to the difference of note observed by Mr. Birmingham 

 (antea, p. 76) hardly applies to the case in question. 



Alfred Newton 



Magdalene College, Cambridge, June 6 



I HAVE been in the habit of observing the notes of cuckoos, 

 and have noticed that the musical interval is very variable. It 

 is not always, or even often, amenable to our tempered scale, 

 but may lie anywhere between a major 2ad and a 4th. The 

 major 3rd seems to be about as frequent as the minor. The 

 interval may vary in the same bird, as it is well known that the 

 cuckoo's song alters greatly with the approach of summer. 



Fr.\xic J. Allen 



St. John's College, Cambridge, June 6 



Cup-marked Stones 



On a large block of fine-grained hard %\-hitish sandstone near 

 Burghead, Elgin, are forty- four cup-marks of various sizes, but 

 all very finely formed. Four of the cups have channels or 

 grooves of various lengths and running in different directions, 

 but none to the edge of the stone. Five have one ring, and 

 channels of various lengths, and in different directions. Four 

 have got two rings and channels, and one has three rings and 

 a channel. In some cases the rings are not complete, thit is, 

 they stop s'aort on either side of the channel, but close to it. 

 One cup has a simple ring. 



From this example, and if I recollect the figures in Sir J. Y. 

 Simpson's work, there seems to be but few cases in which the 

 channels run to the edge of the stone. 



Out of a considerable number of cup-marked stones partly on 

 finely ice-polished rock surfaces and partly on detached blocks 

 large and small, in Elginshire, this is the only one that has 

 rings and grooves. A full description of these, with plans, I 

 have nearly ready to lay before the Society of Antiquaries at one 

 of their early meetings of next session. James Linn 



Keitli, June 2 



The Dumas Number. — In reply to numerous inquiries we 

 may state that the portrait of M. Dumas should form the frontis- 

 piece to vol. xxi., and the article by Dr. Hofmann be placed 

 after the index in the beginning of the volume. 



ENERGY AND FORCE' 

 rY~\N March 28, 1S7J, Clifford delivered a Friday 

 L^-^ evening discourse on this subject at the Royal 

 Institution. By some accident no trace of it, not even 

 the date or title, appears in the printed Proceedings. 

 Thus the lecture escaped notice when Clififord's literary 

 and scientific remains were collected in the summer ol^ 

 last year. A few weeks ago I lighted on my own rough 

 notes of it taken down at the time, probably the only 

 record now in existence. These I have written out, with 

 only so much alteration and addition (indicated by square 

 brackets) as necessary to make them intelligible. The 



' An unpublished discourse by the Lite Prof. Clift'-rd. Wilh an introduc- 

 lor>'na= by J. f. Moiil(tn. 



paper thus produced has been seen by Clifford's friend 

 and mine, Mr. J. F. Moulton, who (besides his general 

 competence in mathematical physics) was thoroughly 

 acquainted with Clifford's mathematical work and ideas. 

 Mr. Moulton has added, by way of introduction, some 

 remarks founded on this intimate knowledge, which will 

 explain the aims of the discourse and supplement the 

 too meagre report which is all that I am able to recon- 

 struct from my notes. — F. POLLOCK.] 



This lecture was, I think, written as a protest against 

 certain loose ideas that had become prevalent relating to 

 energy, motion, and force. The discoveries as to the 

 equivalence of the many forms of energy and the in- 

 variability of the total of energy in any system not 

 operated on by external forces (one case of which is the 

 whole material universe), had led philosophical writers 

 and others to treat force as an entity with a separate 

 existence like inatter, and also, like it, indestructible. 

 The error of thus treating force as an entity with a 

 separate existence was not an unnatural one in those 

 who had not much acquaintance with the theories of 

 physics. No idea is more consonant with the ordinary 

 modes of thought than that force is a something 

 operating from without on a body, and producing 

 effects thereupon in the shape of an alteration of its 

 motion, so that the quasi-personification of force contained 

 in the above does not appear to be in any way an un- 

 warranted conception. The further step, which ascribes 

 to force an indestructibility as absolute as that of matter, 

 is due to a confusion in the terms used by mathematicians 

 themselves in speaking of these subjects, for which 

 they are to blame. Before the conservation of energy was 

 fully formulated, mathematicians were acquainted with a 

 particular case of the general principle, and it had received 

 the name of conservation of force. This unfortunate 

 appellation, with all its misleading tendencies, was often 

 applied to the general principle when the latter first 

 became known, and hence unscientific writers naturally 

 assumed that force and energy were convertible terms 

 and that they were alike indestructible. These erroneous 

 conceptions had attracted Prof. Clifford's attention, and 

 with his usual zeal for preserving scientific ideas from all 

 taint, he set about correcting them. His mode of doing 

 so is highly characteristic. He strikes straight at the 

 root of the matter, and would have us at once cease to 

 think of force as an entity at all. Indeed he goes so far 

 as almost to warn us against tolerating the conception 

 of a cause as distinguished from its effects. 



All we know as to force and motion, he says, is 

 that a certain arrangement of surrounding bodies pro- 

 duces a certain alteration in the motion of a body. It 

 -has been usual to say that this arrangement of sur- 

 rounding bodies produces a certain force, and that it is 

 the action of this force that produces the alteration of 

 the motion. Why have this intermediate term at all ? Why 

 should we not go at once from the surrounding circum- 

 stances to the alteration of motion which follows ? The 

 intermediate term is only a mental inference either from 

 the existence of the surrounding circumstances or from 

 the occurrence of the alteration in the motion ; and if we 

 only accustom ourselves to pass from one to the other 

 without its assistance, it will cease to be necessary, and 

 like other useless mental conceptions, be gradually for- 

 gotten. .And with it will pass all tendency to give to this 

 useless mental phantom any such real and material 

 qualities as indestructibility. 



I was not pre cnt when the lecture was given, nor do I 

 know otherwise than from these notes how Prof Clifford 

 carried out these ideas. But in conversation he had often 

 discussed the matter with me, and made me fully 

 acquainted with his views on the subject, so that I am 

 able thus far to confirm the accuracy and completeness 

 of these notes. It will be seen that he defines force as 



