572 SCIENTIFIC RECORD FOR 1883. 



in connection with Sir William Thomson's lecture noted above. If, as 

 he states, the size of the molecules of this liquid is between 2X10 -8 

 and lx LO -8 millimeters, it follows that the thinnest film measured by 

 the authors, 7.2x10"" millimeters, must contain not less than 3 nor 

 more than 720 molecules in its thickness. As to the surface tension 

 in Plateau's glyceric liquid, the authors give it as about 57 dynes per 

 linear centimeter. Assuming - that the thinnest film measured had for 

 its thickness twice the radius of molecular attraction, the average stress 

 parallel to the surface must be 1.(5x10* dynes per square centimeter, or 

 eight times greater than is required to tear brick asunder. If this 

 radius is the same for all substances, the stress in the surface of mer- 

 cury in contact with air must be nearly ten times greater than in the 

 glyceric liquid, or one-fifth of the tension required to rupture steel bars. 

 [Nature, June, August, 1883, xxvni, 142, 389.) 



The Adams prize essay of the University of Cambridge for the year 

 1S82, on the subject of the motion of vortex rings, written by J. J. 

 Thomson, has been published. It continues the work already done in 

 this direction by von Helmholtz and Sir William Thomson, and carries 

 the theory of vortex atoms to such a stage that in certain general respects 

 it may be applied to the theory of gases. Indeed, the author concludes 

 that the accurate observation of the phenomena of thermal effusion will 

 enable a decision to be reached between the vortex atom theory and 

 the ordinary kinetic theory of gases. (Nature, December, 1883, xxix, 

 103.) 



The meaning of the word "force" continues to be the subject of dis- 

 cussion. Lamb suggests that the true and proper basis of statics is to 

 be sought for in the principles of linear and angular momentum. Two 

 forces are equivalent only when they produce the same rate of change 

 of momentum in any assigned direction, and the same rate of change of 

 moment of momentum about any assigned axis. Two sets of forces are 

 in equilibrium when they produce no effect on either the linear or the 

 angular momentum of any system. Close points out the necessity of a 

 more careful distinction between force proper — the timerate of change 

 of momentum or the space-rate of change of energy — and what he calls 

 impulsion; the former being represented by P and the latter by fF di ; 

 or, assuming F constant, by Ft. Tait, in a paper on the laws of mo- 

 tion presented to the Royal Society of Edinburgh, maintains that force, 

 being simply the time-rate of change of momentum, has no real or ob- 

 jective existence whatever, matter and energy alone being the only 

 objective realities. (Phil. Mag., March, April, November, December, 

 1883, V, XV, 187, 248; XVI, 387, 439.) 



Smith has suggested a modification of the ordinary ergometer (dyna- 

 mometer). It consists of a hollow shaft carrying three pulleys; one of 

 these is loose, one is fast to the shaft, aud the third is geared to the 

 second by three miter-wheels. The two of these whose axes are perpen- 



