PHYSICS. 437 



for weights in a line cutting the line of the knife-edges as nearly as 

 possible, and of course approximately perpendicular to this line, and, 

 for convenience of putting on and oft" weights, hang, as in an ordinary bal- 

 ance, two very light pans, by hooks, on these edges, in the usual way. 

 Now, with the tly-wheel not running, adjust, by weights in the pans, if 

 necessary', so th.it the frame- work rests in equilibrium in acertain marked 

 position, with the axis of rotation inclined sligiitly to the horizontal, in 

 order tliat the axis of the fl^'-wheel, whether si)inning or at rest, may 

 al^'ays slip down so as to press. on one and not on the other of the two 

 end plates belonging to its two ends. Now unhook the pans and take 

 away the gyrostat and si)iu it, replace it ou its knife-edges, hang on the 

 two pans, and tind the weight required to balance it in the marked posi- 

 tion, with the lly-wheel now rotating rapidly. This weight gives an 

 accurate measure of the vertical component of the earth's rotation. 

 {Nature^ September, 1884, xxx, 524.) 



Eespighi has made a series of experiments at the Eoman Observatory 

 in order to determine the intensity of gravity. For this i)uri)ose he 

 used a ])eudulum composed of a sphere of lead O.llGDS meter and weigh- 

 ing 9.43515 kilograms, suspended to a steel wire varying in length in 

 the difterent experiments from 5 to 7.8 meters. The total weight of the 

 wire and the other accessories was only yo^ of the weight of the lead 

 sphere. A correction of 3.4™" was, however, necessary, in consequence 

 of the rigidity of the wire, between the length of the actual pendulum 

 and that of the equivalent simple pendulum. The oscillations were 

 determined by the method of coincidences, a mercury surface being 

 raised at the proper time until a point attached to the pendulum made 

 contact witli it at each oscillation, and made an electro-chronograi)hic 

 record. The results gave for the value of the seconds pendulum 

 0.9934891 meter, and for the value of g 9.805343 meters. These figures 

 difier by only the yttoo P^^t of their value from those calculated by 

 Biot— 0.9933380 and 9.803851. {Atti Ace. Lined, III, xii, 346 ; J. Phys., 

 February, 1884, II, iii, 95.) 



An article by Hall upon inertia has called out replies from Hastings, 

 Mendenhall, and others. Hastings says that the difliculty in the correct 

 use of the word arises from the fact that it has been used in two perfectly 

 legitimate senses — one qualitative, the other quantitative. In the qual- 

 itative sense it simply implies the truth of Newton's lirst law of motion; 

 in the quantitative sense it is mass and nothing else. On account of 

 this ambiguity all careful writers and teachers have long since practi- 

 cally abandoned it. {Science, iii, 482, 559, April, May, 1884.) 



MECHANICS. 



1. 0/ Solids. 



Dobbie and Hutcheson have described an apparatus A.r the rai)ul de- 

 termination of the specific gravity of solids, founded on the measure- 

 ment of the volume of water displaced. It consists of a U tube, one 



