January 27, 192 1 J 



NATURE 



711 



it would need nothing more to imbue the whole pen 

 with the idea that there was some cause for fear. 

 Then they would all make a rush, and their terror 

 and the momentarily recurring incentives to, and 

 aggravations of, it in the shape of collisions would 

 only subside when the sheep had broken out and were 

 in the open, clear of one another and of their troughs 

 and hurdles. 



If this is the explanation of the panic, then it is 

 easy to understand why folded sheep are so much 

 mQre likely to suffer than those lying in open fields. 



The heavy, oppressive atmosphere accompanying the 

 thick darkness, the susceptibility of sheep to atmo- 

 spheric disturbance, and their nervous and timid dis- 

 positions would all tend to increase the fright the 

 sheep experienced. The cause of the panic being a 

 cloud foiling along so low down as (apparently) to 

 touch the ground, the tops of the hills arid the highr 

 lying ground would naturally be most affected ; and this 

 is observed to be the case, although locally the usual 

 direction follow'ied by thunderstorms has indicated a 

 line along which sheep stampeded on nearly every farm. 



The Work and Discoveries of Joule.' 

 By Sir Dugald Clerk, K.B.E., F.R.S. 



T"* HE greatest generalisation in the early history of 

 *■ physical science was made late in the seventeenth 

 century by Sir Isaac Newton when he enunciated the 

 laws of motion and deduced from them the existence 

 in space of attraction between planets and the sun. 

 Mechanical science has been built up on Newton's 

 fundamental propositions and discoveries. The dis- 

 covery by Joule in the middle of the nineteenth 

 century of the mechanical equivalent of heat and his 

 suggestion and determination of the existence of an 

 absolute zero, together with the adaptation of Carnot's 

 lycle of 1824 to the theory of heat as a mode of 

 motion, provide generalisations of equal importance 

 to Newton's law of gravitation, and from them funda- 

 mental thermodynamic laws are deduced : the equi- 

 valence of energy in different fcms, conservation 

 i>f energy and dissipation of energy. Joule's dis- 

 covery, in fact, called the modern science of thermo- 

 dynamics into existence. 



Manchester has been the home of many highly 

 distinguished men — great scientific men, great in- 

 ventors, and great masters of industry and business — 

 but it is fortunate indeed in its connection with two 

 of the greatest discoverers in the history of the world, 

 Dalton and Joule. Joule read his first paper before 

 ihc Manchester Literary and Philosophical Society in 

 the vear 1841 upon the subject of "The Electric 

 Origin of the Heat of Combustion." He contributed 

 n long scries of papers from that year until 1879, a 

 period of thirty-eight years, and he dealt with a great 

 variety of subjects, including experimental investiga- 

 tions on the phenomena of the voltaic current, the 

 •Irtermination of the specific heat of bodies, heat and 

 institution of elastic fluids, mirage, freezing point of 

 hormomrters, galvanometers, dip circle, solar photo- 

 raphs, duty of electro-magnetic engines, magnetic 

 lorms, polarisation of platinum plates, mercurial air- 

 umps, and telescopic oscillations. 



The debt of the practical engineer to Joule and 

 his great associates is very real, but the science of 

 thermodynamics did not supply the fundamental laws 

 from which heat-engines were invented and developed. 

 The steam-engine had been developed by Newcomen, 

 Sme.iton, and James Watt long before the birth 

 of the science of thermodynamics. What is true of 

 the steam-engine is true also of the hot-air engine 

 and the internal-combustion engine ; all the known 

 types of heat-engine at present in use were invented 

 before the year 1850, and practical experimental 

 examples of both hot-air and internal-combustion 

 Engines were then in operative existence. Thermo- 

 dynamics supplied the laws of the conversion of heat 

 into mechanical work by which these engines are 

 governed ; it explained the relative perfection of 

 engines already in existence, but it did not create 

 these engines. It performed the very important ser- 

 vice of dispelling the errors of thought which hindered 

 the future advance of heat-engines Such errors as 

 to the theory of the regenerator and the theory of 

 compression and expansion in all steam and internal- 

 combustion engines, held by the most eminent en- 

 gineers and scientific men so late as from 1845 to 

 185.^, were rendered impossible by the splendid work 

 of Joule, Kelvin, Rankine, and their Continental col- 

 leagues. The knowledge of thermodynamics has thus 

 an increasing effect upon instructed engineers of the 

 present generation. It is quite obvious that although 

 the origin of heat-engines cannot be ascribed to 

 Joule's work, yet the imorovemcnt and final develop- 

 ment towards a maximum conversion of heat 

 into mechanical work are rendered possible to 

 thei engineer of to-day by his great discoveries. 

 Engineers and engine-designers are most grateful 

 to Joule, and look back on his achievements 

 as those of llv utmost intellectual and practical 

 importance. 



Giant and Dwarf Stars.* 



THE amount of light received from 

 mines its apparent magnitude (m), 

 two stars differing by one magnitude 

 I'he absolute niafinitude (M) is what 

 magnitude would be if the star were at 

 distance of 10 parsecs, which corresponds 

 of 01'. H IT is the parallax of a star 



M-m+s+slogir- 



> Abalraet o4 the ftr^t Ioal« Mcffwriiil l.«ctitr« detivrrrd on TtiMday, 

 r>co«mb€r 14, i9to, to th« M«nch«atcr IJurmry and Philovjptikal .Society. 



* Ab«tr«ct 01 a l«clttr< rfaltvared befora the Royal Society of Victoria, 

 Melbourne, on October 14, I9Ms by 1)t, }. M. ItaMMrin, Cnrmmmfnl , 



a Star deter- 

 the ratio for 

 being 8-512. 



the apparent 

 the standard 

 to a parallax 



in seconds of 



NO. 2674, VOL. 106] 



In this equation m is not difficult to measure, and 

 hence if ir or M is determined the other can be found. 



Russell took all st.nrs for which fairly accurate 

 values of «■ were available, and from the above equa- 

 tion computetl M. Then, plotting M as ordinate and 

 type of spectrum as abscissa, he found th,lt (i) all 

 wnite stars are far brighter than the sun ; (2) range 

 of brightness increases with redncs.s ; (3) all faint 

 stars are red ; and (4) all red Mars are very bright or 

 very faint. 



Adams and KohlschOtter found that the relative 

 intensity of srlrcte<I lines in the spectrum of a star 

 depended on the absolute magnittide from measure- 

 mertt» ort the spectrum. M beinif determined, the 



