148 



SCIENCE. 



[Vol. XIX. No. 475 



librium. Rest is the opposite of motiou ; it is the state of 

 matter in which the physical forces acting on it are in equi- 

 librium; that is when the force impelling motion in a given 

 direction is counteracted by an equivalent force impelling 

 motion in the opposite direction ; or is resisted by a superior 

 force. A stone rests on the surface of the earth because the 

 force of gravity acting on the stone is resisted by the force 

 of cohesion in solid matter; but the force continues although 

 there is no motion resulting from it. The stone sinks in 

 water, that is, it moves froai the force of gravitation because 

 the force of cohesion in the molecules of water is insufficient 

 to counteract the force impelling motion ; but when the force 

 of cohesion in the molecules is sufficiently increased by con- 

 gellation, the stone rests on the surface of the ice. So a top 

 spun rapidly rests on its peg, because the force giving it hori- 

 zontal motion counteracts the pull of gravity which causes 

 it to fall when the rotation ceases. 



Dr. Mayer defines force as " Somet'ning which is expended 

 in producing motion: and this something which is expended 

 is to be looked upon as a cause equivalent to the effect, 

 namely to the motion produced." ' 



This is obviously too narrow to include even dynamic en- 

 erjry. Two horses pulling a vehicle in opposite directions 

 with the same force would produce no motion; divide the 

 force by unhitching one of the horses, and the vehicle moves. 

 Then, according to this definition, we have the absurdity 

 that the whole force is nothing, but half of it is something. 



A correct definition of phpsical force is that it is some- 

 thing producing the state of ponderable matter in which it is 

 subject to human observation. Whether the state be one of 

 motion or rest, hot or cold, solid, liquid, gaseous, colored, 

 etc., it is the result of force. We only know physical force 

 from its effects on ponderable matter, and we only know 

 ponderable matter as affected by force. 



The supposed difficulty in the concept of an element in 

 nature entirely distinct from, but inseparably connected with, 

 ponderable matter, is entirely, factitious. Time and space 

 are such elements, entirely distinct from, and inseparably 

 related to, ponderable matter; and the concept of force as 

 above defined is as absolute and imperative as the concept of 

 time, the concept of space, or the concept of matter itself. 

 The progress of science in tracing a force through its various 

 manifestations, as has been done to some extent with gravity, 

 confirms the primal concept of force which comes with the 

 very dawn of intelligence. 



The still more abstract concept of law by which any force 

 is what it is, is also primal, absolute, and inevitable in every 

 human intelligence. 



Whether all ponderable matter is one as claimed by some 

 philosophers, or whether all force is one as claimed by other 

 philosophers, are speculations which, with our present 

 knowledge of these elements, are idle if not mischievous. 



It is undoubtedly from plienomena resulting from the ap- 

 parent differences in ponderable matter, and the apparent 

 differences in the forces acting on it, that real progress in 

 unravelling nature has been made. 



We need a specific name for this force of which molar 

 motion, molecular motion, heat, and light, are manifesta- 

 tions. There seems to be no doubt that positive electricity 

 is also one of its forms. Electricity, like heat, is developed 

 by friction and by chemical reaction; and its mechanical 

 equivalent, or, more accurately, the electric equivalent of 

 molar motion, doubtless is the same as the heat equivalent of 

 molar motion, or differs from it by some law which will 



= "Correlation and Conservation of Forces," D. Appletou & Co., 1S90, p, 335. 



pi'ove the identity of the force. Dr. Mayer suggested that 

 whether friction, which of course is resisted molar motion, 

 developed heat or electricity, depended on the character of 

 the substances used in the friction, homogeneous substances 

 developing heat and heterogeneous substances electricity. 

 There appears lo be no essential difference in the chemical 

 reactions which develop heat and those -which develop elec- 

 tricity; the difference apparently being in the mode of ap- 

 plying the force or energy and the substances to which it is 

 applied. 



Electricity passes from dynamic to potential under not 

 precisely the same conditions as heat, but not more essen- 

 tially different than the conditions under which motion 

 passes from dynamic to potential, and its dynamic power is 

 exhausted in doing vpork. This feature of electrical energy 

 has been utilized by Mr. Hodges in his new lightning-rod, 

 constructed of copper ribbon, so arranged that the copper 

 will be dissipated by the electric current. 



But I must leave this branch of the subject to those better 

 informed as to the phenomena. 



There may be still other forces, or rather forms of force, 

 which may be found to have equivalence and mutual con- 

 vertibility with heat. It is equivalence and mutual conver- 

 tibility which warrants the assumption that motion and heat 

 are phenomena resulting from, or, more accurately, are 

 manifestations of, the same force. 



In speaking of the force itself, I have used the expression 

 " force or energy " because these words have several mean- 

 ings, and the sense in which they are synonymous comes 

 nearer the expression of the concept sought to be presented 

 than any other phrase that has occurred to me. But it would 

 facilitate induction if we could call it " Ergic Force," or 

 "Ergism," or give it some other specific designation to dis- 

 tinguish it from other forces, or force generally, including 

 under the term "Ergism" every manifestation of force for 

 which a heat equivalent may be found. This name seems 

 appropriate because it suggests the element in nature which 

 is the basis of work It enables us to grasp a concept of the 

 force distinct from its manifestation in any one of its forms; 

 and if the delimitation itself is correct we can class as 

 " Antergic " the forces, like cohesion, which have no heat 

 equivalent, but which, under certain conditions, render dy- 

 namic "Ergism" potential. Dakiel S. Troy. 



LEXrERS TO THE EDITOR. 



*** Correspondents are requested to be as brief as possible. The writer's name 

 is in all cases required as proof of good faith. 



On request in advance, one hundred copies of the number containing his 

 communication imll *>€ furnished free to any correspondent. 



The editor will be glad to publish any queries consonant with the character 

 of the journal. 



Further Notes on the Loup and Platte Rivers. 



Several jears since it was my privilege to spend several weeks 

 studying the peculiar drainage of central Nebraska I have there- 

 fore been much interested in the papers of Professors Hicks and 

 Davis in recent numbers of Science. 1 trust I shall not be intrud- 

 ing if I call attention, at this time, to a few additional facts which 

 seem to have a bearing on the discussion. 



1. The streams north of the Platte, from Kearney to Fremont, 

 have their courses first quite regularly south-east, then, as they 

 near the Platte, they turn to the east-north-east, adopting the 

 direction of that stream. Not only is this true of the Loup system, 

 as Professor Hicks has well shown, but also of Snell Creek and 

 Maple Creek further east. 



2. There are dry channels, but little above the streams, connect- 

 ing the Loup with Shell Creek, and Shell Creek with the Maple, 

 which are known as Lo3t Creek and Dry Creek. Tbese lie in the 



