822 



SCIENCE 



[N. S. Vol. XXXI. No. 804 



of the several quantities are chosen so as to 

 make the equation true, the amount of the 

 force is numerically equal to {MY^ — MV^ 

 -f- T, or to the rate of change of momentum. 



Let r^l second, 7, = 0, 7, = 32.2, 

 W^l Ih., Jlf = 1/32.2, then the equation re- 

 duces to 1 = 1/32.2 X 32.2, or force = mass X 

 acceleration, and it is correct, but if the unit 

 of M is taken as 1 lb. then we have 1 = 

 1 X 32.2, which is incorrect. 



The " correct, competent and complete " 

 definition that force is the rate of change of 

 momentum, no doubt is a metaphysical de- 

 duction from the formula, but it is neither 

 correct, competent nor complete, and is not a 

 definition at all. It assumes that we can 

 translate the sign of equality (=), which 

 really means " is numerically equal to " by 

 the word " is." It is not true even as to equal- 

 ity except under certain limited conditions, 

 viz., 1, that the units have certain values, such 

 as ilf = lbs. H- 9, and 2, that the body is free 

 to move. It is not true when a force is ap- 

 plied to a body not free to move, nor when a 

 force is being applied to cause a body to move 

 at a constant speed against a constant resist- 

 ance, as when a canal boat is being towed, 

 nor when a force is applied to a body moving 

 with increasing speed with decreasing ac- 

 celeration, as when an engine is bringing a 

 train up to full speed. 



"The debt that physics owes to metaphys- 

 ics " is a sound castigation, for having intro- 

 duced into physics such bad logic as that of 

 making " equals " equivalent to " is," " dark- 

 ening counsel with words," and substituting 

 metaphysical deductions and complex concepts 

 for simple definitions and concepts; and for 

 introducing ideas that are so far from being 

 " clear, sharp and definite " that they have to 

 be unlearned or forgotten before the student 

 can make satisfactory progress in engineering 

 mechanics, and that they are discouraging 

 even the high-school physics teachers them- 

 selves from teaching elementary dynamics, as 

 was shown in Professor Edwin Hall's paper 

 in Science of October 29, 1909. What is 

 needed is a return to the good old definitions 

 of Weisbach and Rankine, and a dropping of 



the metaphysical reasoning which has re- 

 cently become the fashion. Wm. Kent 



MONTCLAIB, N. J., 

 April 26, 1910 



SCIENTIFIC BOOKS 



SCIENTIFIC RESULTS OF SHACKLETON's SOUTH 



POLAR EXPEDITION 



The Heart of the Antarctic: Being the Story 

 of the British Antarctic Expedition, 1907- 

 1909. By E. H. Shackleton, C.V.O. With 

 an Introduction by Hugh Egbert Mill, 

 D.Sc. An Account of the First Journey 

 to the South Magnetic Pole. By Professor 

 T. W. Edgeworth David, E.R.S. 2 vols., 

 ill., plates. Philadelphia, J. B. Lippincott 

 Co. 1909. $10 net. 



It rarely falls to the lot of any single ex- 

 plorer to conjointly arouse such popular in- 

 terest and contribute such important scientific 

 knowledge as has been done by Sir Ernest H. 

 Shackleton through his Antarctic expedition 

 of 190Y-19©9. It should be realized that the 

 inception, financeering, organization and suc- 

 cessful administration of the expedition are 

 due to Shackleton, it being a private venture 

 unaided, and it may be also said unhampered 

 by governmental offices. 



Shackleton played an active part in Scott's 

 Antarctic expedition, 1901-1903 — when he was 

 one of the four men who made a world's record 

 of the farthest south — from which he was later 

 invalided on account of scurvy. His early 

 experiences were fruitful factors in his recent 

 successes, which were in a measure due to 

 improved conditions of food, clothing, shelter, 

 transportation and travel methods. 



Sailing from New Zealand, January 1, 1908, 

 Shackleton established his permanent station 

 at Cape Eoyds, Eoss Island, at the base of 

 Mt. Erebus. The expedition returned in 1909 

 with its members in health and its work done 

 with wonderful success. In addition to the 

 meteorological work at Cape Eoyds, the fa- 

 mous volcano Erebus was ascended and 

 studied, the south magnetic pole located and 

 visited, while the southern party attained a 

 point within 93 geographical miles of the 

 south pole. 



