494 



SCIENCE. 



[Vol. II., No. 36. 



spring l)ahiiice used which measured absolute 

 weight, the dealer who should purchase tea at 

 one latitude, and sell it at another, would be 

 subject to a gain or loss, depending upon the 

 difference in the force of gravitj'. 



It is not, however, on merely commercial 

 grounds that the change rests. For scientific 

 purposes a unit is used as a term of comparison 

 between different quantities of the same kind, 

 and must be so defined and chosen as to fulfil 

 this function with the greatest convenience. 

 Now, a unit of force which sh.all furnish a di- 

 rect and convenient standard of comparison 

 between forces or weights at different places is 

 entirely impracticable. At an}- one place the 

 weight of a given mass of metal may be taken 

 as a convenient unit ; but this unit will change 

 when we go to any other place, owing to the 

 difference in the force of gravity. Indeed, 

 ever}' student of phj'sics knows that the meas- 

 ure of the force of gravity at any one place 

 is one of the most delicate and difficult prob- 

 lems in physics. In the definition w'hich re- 

 fers to the latitude of 45° it is assumed that 

 the force of gravitj' is the same at all points on 

 this parallel. We now know that this is not 

 the case, and that if we adopt such a unit we 

 shall have to define the exact spot on the 

 earth's surface which is taken as the standard. 

 Reference to such a standard woiild be imprac- 

 ticable. Hence a unit of force must be sub- 

 sidiary to the unit of mass. The most conven- 

 ient ■way of fixing it is to take the unit of mass 

 as known, and to determine the force of gravity 

 at the place of observation. The combination 

 of the two gives a standard bj' which weight 

 may be expressed iii force. To be more ex- 

 plicit : if we have a piece of metal the mass of 

 which we know to be one gram, and if we deter- 

 mine the force of gravity at the place to be n, 

 then the gravitating force of that piece of metal 

 will be known to be n units of force. In prac- 

 tice this must be the method used in physics, 

 if an accurate measure of forces is reallj- re- 

 quired. 



Let us now consider M. Jamin's objections. 

 He says that the mass of a bod^- is not suscep- 

 tible of direct determination ; for to mea,sure it 

 we must commence bj' determining its weight 

 in a balance, .and afterwards dividing by the 

 number which expresses the acceleration of 

 gravit}- at the latitude of 45° and at the level 

 of the sea. It is difficult to attribute this re- 

 mark to anj- thing but inadvertence, since the 

 division by g at 45° is necessary oulj" on the 

 French system. If we measure it by means of 

 a balance having grams as weights, the result- 

 ing weight is at once the mass on the C. G. S. 



system, no matter where the weighing is made, 

 and therefore needs no division whatever. 



lie then adds, "Suppose, on the contrary, 

 that we have to measure a force : we determine 

 it directly bj- means of weights at the place of 

 observation. Afterwards wc apply to these 

 weights the corrections relative to the latitude 

 and the altitude, to have an expression of the 

 force as the function of a normal gram. We 

 must remark that we cannot avoid these correc- 

 tions to taking mass as the fundamental unit ; 

 because it is always weights that we measure, 

 and the course followed in the experiments is 

 necessitated bj- the nature of things. ' ' This is 

 quite true, but it does not prove that one sys- 

 tem affords anj' more convenient unit of force 

 than the other. Sniux Newcomb. 



STANDARD RAILWAY TIME. 



The problem of simplifying the sj'stem of 

 time standards used by the railways of this 

 countrjT seems to be near solution. The rep- 

 resentatives of various railway-lines, who are 

 to-day in session at Chicago, will receive the 

 report of the secretary, Mr. W. F. Allen, and, 

 it is expected, will take final action. For some 

 years past, committees of various scientific 

 bodies, as the American metrological societj', 

 the American association for the advancement 

 of science, and the American society of civil en- 

 gineers, have called attention to the urgent need 

 of reform in the standards of time in use, and 

 suggested plans for action. The railways, which 

 are naturally most interested in the movement, 

 have recently taken hold of the matter in ear- 

 nest. The plan which has met with the most 

 favor is that in which five standards of time, 

 differing by consecutive hours, are proposed 

 for the whole territory occupied by the United 

 States and Canada. These are based upon 

 the meridians from Greenwich, but receive 

 other names for purposes of convenience. It 

 is proposed bj- the railways that in Canada the 

 standard shall be known as intercolonial time, 

 and shall coincide with the local time on the 

 meridian four hours, or 60°, west of Greenwich. 

 In the United States the standards will be 

 known as eastern, central, mountain, and Pa- 

 cific time, and coincide with the local times ou 

 the meridians five, six, seven, and eight hours, 

 or 75°, 90°, 105°, 120°, respectively, west of 

 Greenwich. The advantage of this system is, 

 that the standards will differ from the true 

 local times of the various parts of the country 

 bj' amounts not greater than thirty minutes, if 

 the divisions are made rigidly according to 

 longitude, and no one will be inconvenienced 



