334 



SCIENCE. 



[X. S. Vol. XXI. No. 531. 



Let a ray of light of fixed direction fall 

 upon a mirror turning about a vertical 

 axis. The reflected ray is received upon 

 a plane. If the incident ray passes 

 through the vertical axis, and this axis lies 

 in the plane of the mirror, and the plane 

 on which the reflected ray falls is parallel 

 to the mirror-plane, the curve traced by 

 the spot of light as the mirror turns is a 

 hj-perbola. Discussion of another case. 



Some Convenient Laboratory Apparatus: 

 Horace C. Richakds, University of 

 Pennsylvania. 



I. An Apparatus for Quickly Washing 

 and Filling Vessels icith Small Openings. 

 — It consists of a small glass cylinder 

 mounted on a suitable stand and provided 

 with three outlets, one leading to the vessel 

 which is to be filled, one to that containing 

 the liquid, and the third to an aspirator 

 through a trap. The liquid is first drawn 

 up into the cylinder and then, by inter- 

 mittent working of the aspirator, is driven 

 into the vessel. A similar process empties 

 the vessel and carries the liquid over into 

 the trap. For mercury, etc., the form is 

 slightly modified so as to prevent loss of 

 the liquid. 



II. A Simple Automatic Mercury 

 ^yasher. — Mercury is raised by an as- 

 pirator into a vessel placed above a vertical 

 tube filled with dilute nitric acid. AVlien 

 the vessel is filled, the aspirator is auto- 

 matically shut off and at the same time 

 air is admitted by a pair of valves operated 

 by a float. The mercury then filters 

 through a small valve in the bottom of the 

 vessel and falls in small drops through the 

 column of acid. It is then returned to the 

 reservoir from which it was drawn and so 

 is passed around through the acid as often 

 as is desired, the aspirator being set in 

 action again when the upper vessel is 

 nearly empty. 



The Double Suspension Pendulum for De- 

 termining the Absolute Value of the 

 Acceleration of Gravity : R. S. Wood- 

 ward, Carnegie Institution. 

 This paper describes an apparatus spe- 

 cially designed to avoid the difficulties 

 presented by knife-edge pendulums and to 

 secure a degree of precision in absolute 

 measures of the acceleration gravity com- 

 parable with the precision already attained 

 in relative measures. 



Seat Insulation of Observatory Domes, 

 Laboratories and Other Buildings. 

 David Todd, Amherst College. 

 To prevent excessive heat accumulation 

 in the new observatory domes at Am- 

 herst waste granulated cork is put in be- 

 tween the interior galvanized iron sheath- 

 ing and the exterior wood boarding on 

 Avhieh the copper roof is fastened. One 

 and one half to two inches of cork is suffi- 

 cient to keep interior iron always cool to 

 the touch, no matter how hot the copper 

 gets when the sun is shining normally 

 upon it. Mineral wool would be nearer 

 fire-proof and equally good insulation, but 

 adds more weight. Numerical tests will 

 be submitted. 



The Relation Between Air Pressure and 

 Velocity. Francis E. Nipher, Washing- 

 ton University. To be published in 

 the Transactions of the Academy of Sci- 

 ence of St. Louis. 



The paper describes a method of de- 

 termining the constant in the equation 

 P = kv'^ when the air blows into the open 

 end of a tube collector. The tube was 

 bolted to a small pulley on a shaft which 

 could be run at various speeds. The open 

 end was 36 inches from the center of the 

 pulley, and the plane of the opening could 

 be set at any angle Avith the cix'cular path 

 which it describes. When at right angles 

 to the path the air within the tube is found 

 to be in o(iuilibi-iuni during rotation. The 



