8i4 



NATURE 



[August 25, 192 1 



ment demonstratif de 1 'installation nouvelle de 

 50,000 chevaux en cours de montage a la Mil- 

 waukee Electric Railway and Lighting Co. 

 destinee a alimenter une centrale de 200,000 kw." 

 At home pulverised coal has recently been applied 

 at the Hammersmith Central Electrical Station. 



The advent of a new process in connection with 

 coal dust has resulted in a considerable step for- 

 ward being made towards the reduction in the 

 extent of the equipment necessary in the prepara- 

 tion and conveyance of coal dust for combustion. 

 This process is that by which the finely divided 

 coal dust is intimately mixed with oil to form what 

 is inaccurately termed a "colloidal" fuel, for 

 colloidal it is not. In this process the coal is 

 ground in oil, a mixture resulting which is suffi- 

 ciently stable for all practical purposes, espe- 

 cially so when the proportion of' sbtid fuel con- 

 tained therein exceeds 50 per cent. ; mixtures of 

 equal quantities of oil and coal have been used 

 after standing three months in barrels without any 



difficulty having been experienced in regard to 

 sediment. 



In the case of the so-called "colloidal" fuel> 

 unless the amount of moisture is very excessive, 

 the coal can be used without having to resort to 

 drying preliminary to crushing, which means a 

 curtailment in the equipment required as com- 

 pared with the use of simple pulverised fuel. It 

 has a further advantage in respect of transporta- 

 tion and of handling, in that it is a semi-liquid, 

 and can be treated as an oil fuel, after due 

 allowance for its greater viscosity. It is not 

 liable to spontaneous combustion, and is burnt in 

 the same manner as if it were " straight " oil. 



The field for the use of "colloidal " fuel is great. 

 The fuel can be employed wherever oil is applic- 

 able as a steam raiser. Its wide application will 

 result in a vast saving in the consumption of oil, 

 and its manufacture allows of the useful employ- 

 ment of low-grade coals and of coals deficient, for 

 other purposes, in volatile constituents. 



€marks on Gravitational Relativity.^ 

 By Sir Oliver Lodge, F,R.S. 



IV/ 



WHEN we come to the more general theory, 

 which attends to the acceleration and not 

 merely the velocity of the observer, I find myself 

 in disaccord on some points with many eminent 

 exponents, chiefly in connection with their aboli- 

 tion of the idea of " force," and the consequent 

 replacement of gravitation by a modified geo- 

 metry ; as if the earth's natural motion was in a 

 hypocycloidal sort of spiral, and was not under 

 compulsion by any deflecting force. 



A revolt against " force " as a real objective 

 entity was led by that great mathematician and 

 physicist, Prof. Tait of Edinburgh. In the first 

 instance he rebelled against the practice, adopted 

 by text-books of the period, of using the term 

 " accelerative force " instead of " acceleration," 

 and making a muddle of the laws of motion by 

 formulating what they called Law 3 thus : — 

 " When pressure communicates motion to a body 

 the accelerative force varies as the ratio of the 

 pressure to the mass." Then he objected to some 

 of the pedagogic arrow-heads sprinkled on 

 mechanical diagrams, especially the arrow-head 

 representing centrifugal force ; since it is obvious 

 that no such force acts on the revolving body. 

 Ultimately Tait or his disciples (W. K. Clifford 

 too, if I remember right, also Mach and Kirchhofi") 

 were prepared to abandon the term force alto- 

 gether, and to substitute space-rate of change of 

 energy, or time-rate of change of momentum, or 

 mass multiplied by acceleration, as a more real 

 equivalent. Tait even denounced the idea of 

 balanced forces, saying that only their effects were 

 balanced (" Ency. Brit.," 9th ed., art. " Me- 

 chanics," §§ 285-300); as if two opposing forces 



1 Continued from p. 785. 



NO. 2704, VOL. 107] 



were each producing their proper amount of 

 acceleration, or of momentum, but in opposite 

 directions. Though how this kind of statement 

 could include the production of scalar quantities, 

 like work and energy, is not apparent. The whole 

 idea of *' cause " came into disrepute. 



Now mass-acceleration truly is a measure of 

 the force which produces it, but that does not 

 mean identity. Reformers spoke sometimes as if 

 they meant identity, and desired to get rid of the 

 term force altogether because it had been so 

 misused. After a lecture by Prof. Tait to the 

 British Association on " Force " (at Glasgow, in 

 the year 1876), Sir Frederick Bramwell amusingly 

 said that in the North of Britain the term meant a 

 waterfall, while in London it meant the police, 

 and that really, after the lecture, he himself 

 scarcely knew exactly what it did mean ! In that 

 lecture Tait had dealt pugnaciously with some 

 misuses of the term by Prof. Tyndall and other 

 scientific people ; for it is not so long ago that 

 the words vis and Kraft were used with but little 

 modification or caution for the quite different con- 

 ception of Energy, " The Persistence of Force " 

 was a phrase frequently employed in philosophic 

 writings. Indeed, an accurate nomenclature has 

 scarcely yet penetrated into common usage ; and 

 the result is an unnecessary vagueness about the 

 term, typified by Sir F. Bramwell's more than half 

 serious confession. Centrifugal force, for example, 

 can be treated correctly enough by equating it to 

 the product of inertia and rate of change of velo- 

 city, but that does not do away with the force : 

 the force is exerted by the revolving body against 

 its constraints. The word is misleading if thought 

 of, in what was no doubt its original intention, as 

 a radial fly-away tendency ; it should connote only 



