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XCVII. Notices respecting New Books. 



Coins dephysiqus generale. Par H. Oluvieh. (Paris : Librairie 

 Scientifique. J. Hermann, 1921.) 



'T^HIS tome premier is devoted to a discussion of units, gravi- 

 -*- tation, electro-magnetic, in the C. G. 8. system, and a new 

 M.T.S. system (metre-tonne-second) recommended here. 



What is the matter with Physics Training for Engineers ? 

 This is the question being asked to-day. The answer is sure to 

 be : It is the C.G.S. source of Arrogance. 



These niggling microscopic units are thrown aside by the young 

 engineer as soon as he is free from the tyranny of the lecture and 

 examination room. 



They are described by Halsey in his 'Handbook for Draftsmen' 

 as a " Monument of scientific zeal with ignorance of practical 

 requirements. The object of Weights and Measures is to Weigh 

 and Measure, not merely to make calculations.'"' 



The second is acclaimed as the unit of time, because it keeps g 

 down to an easily remembered number for calculations. But the 

 engineer prefers the minute to record revolutions, as an interval 

 that can be cheeked with accuracy on a stop-watch. And in the 

 astronomical units of Relativity the unit of time is nearly 1000 

 years. 



But when it came to the choice of such diminutive units as the 

 centimetre and gramme, in preference to the commercial metre 

 and kilogramme, the decision was made for the supposed con- 

 venience of making the density of water unity, so that density and 

 specific gravity would be the same number, and a name would be 

 saved. 



On the M.K.S. (metre-kilogramme system) the density of 

 water is lOuO fkg/m 3 ), with the advantage of keeping the allow- 

 ance for buoyancy of the air in view, as a correction of the last 

 figure, say a deduction of 1*25 ; and so a Table of Density would, 

 if absolute, require the same deduction of 1*25 to give apparent 

 density in air. But this correction is out of sight in the C. G. S. 

 system, and we never hear it mentioned, although an accurate 

 measurement must be carried out by a human being in an atmo- 

 sphere where he can breathe, and not in vacuo. 



The same theoretical pedantry has influenced our author in his 

 selection of the M.T.S. system, with the same view of keeping 

 specific gravity and density the same figure, to the same decimal, 

 but usually ignoring the decimal when air buoyancy makes itself 

 felt ; paramount in the balloon. 



A writer on Hydrostatics is equally loose when he tells us to 

 neglect the pressure of the atmosphere. He should observe the 



