PHYSICS. 469 



pendent of the pressure. {Comptes Rendus, January, March, xciv, pp. 

 101, 822; Nature, May, xxvi, p. 44.) 



Colladon shows to his classes the resistance of the air in guns in the 

 following way : The reservoir of an air-gun is fully charged with com- 

 pressed air, and the gun is loaded as usual, the ball nearly fitting the 

 bore. Placing the barrel vertical, and closing the mouth firmly with 

 his thumb, the gun is fired by an assistant ; the thumb remains in posi- 

 tion and the ball is heard to fall back in the barrel. Then, using the 

 same ball, he fires with the same gun and pierces a pine board 0.4 inch 

 thick. If the barrel of the gun is more than 32 inches long, and the 

 ball is spherical and nearly fills the tube, and if the operator is sure of 

 the strength of his thumb, the experiment is without danger. {Nature^ 

 August, XXVI, p. 353.) 



Melsens has made a study of experimental ballistics, with a view to 

 determine the influence of the air. He finds that the air which accumu- 

 lates in front of the projectile forms a layer there capable, in the case 

 of high velocities, of preventing the immediate and absolute contact of 

 the two solid bodies, especially at the point of impact, the incidence 

 there being normal. {Ann. Chim. Phys., March, V, xxv, p. 389.) 



Stroh has repeated the experiments of Bjerknes on vibratory attrac- 

 tion and repulsion, but has made use of air instead of water as the 

 medium. Moreover, he has investigated the condition of the air between 

 the vibrating drums by inclosing these in a closed space furnished with 

 an alcohol manometer. When the phase of vibration was the same for 

 the two drums, the manometer showed an increase of pressure, the air 

 being expelled from between them ; while when the phase was opposite, 

 the alcohol rose in the manometer tube. Further, the space between 

 the drums is crossed by lines of pressure practically identical in direc- 

 tion with those which Faraday showed in a magnetic field by means of 

 iron filings. {Nature, June, xxvi, p. 134.) 



Kraievitsch has shown that the rarefaction obtained by means of a 

 mercury pump is limited, the air being replaced by mercury vapor, the 

 tension of which is a function of the temperature but at ordinary tem- 

 peratures is 0.02 millimeter. To show the influence of cold on the 

 vacuum he made a Geissler tube with a cylindrical appendage of glass, 

 the whole being exhausted by means of steam, as in the case of the 

 water hammer. At ordinary temperatures the spark passed as a 

 luminous line; but on cooling the appended cylinder to --200 the light 

 became stratified and filled the tube. He doubts the possibility of get- 

 ting a vacuum as high as 0.00004 meter, as Crookes claims, since the 

 McLeod gauge measures only the elasticity of a permanent gas. The 

 author describes some improvements which he has made in MendeleefPs 

 pump. (J. Soc. Phys. Ghim. Busse, xiii, p. 335 ; J. Phys., December, 

 II, I, p. 578 ; Nature, February, xxv, p. 377.) 



Chancel has contrived a simple apparatus for determining the density 

 of gases in order to fix their molecular weight as a control in chemical 



