C. GENERAL PHYSICS. 127 



radiation. Messrs. Whitehouse & Latimer Clark, an electrical 

 recorder for registering time, speed, distance, and number 

 of passengers inside and out of tram - cars and omnibuses. 

 This information is registered in four parallel columns, by 

 automatic pens, in red ink, on long strips of paper. Dr. Tyn- 

 dall exhibited the apparatus constructed by himself and his 

 assistant, Mr. Cottrell, for showing the stoppage of sound by 

 a non-homogeneous mixture of air and vapors, and also ex- 

 periments illustrating Savart's observations on the action of 

 sound on a jet of water. Mr. J. Norman Lockyer exhibited 

 a series of photographs of metallic and solar spectra, enlarged 

 from photographs taken by his new method of comparing 

 spectra by means of a perforated shutter sliding in front of 

 the slit of the spectroscope. Captain J. E. Davis exhibited a 

 sextant which will be found particularly useful in night ob- 

 servations, as it permits the taking of a series of observations 

 without reading off any until the close of the series, this be- 

 ing accomplished by the adaptation of a micrometer move- 

 ment to the tangent screw, and the application of indicators 

 to the arc of the instrument. Messrs. Tisley & Spiller ex- 

 hibited their compound pendulum apparatus in action, and 

 distributed cards with the exquisite curves described upon 

 them. Mr. E. B. Tylor's ingenious apparatus for illustrating 

 refraction was shown, and Mr. Spottiswoode's triple com- 

 bination double-image prism. Messrs. ISTegretti & Zambra 

 exhibited their new ingenious upsetting thermometer, for 

 recording temperatures. Mr. G. P. Bidder's micrometer is 

 described as a most ingenious device for observing the tran- 

 sit of very faint stars. The spider lines are illuminated by 

 a side light, and are reflected into the eye-piece by a mir- 

 ror, as bright lines upon a dark ground, and can be colored 

 at pleasure by the interposition of colored glasses. The 

 beautiful photometer of Sir Charles Whcatstone is worthy of 

 special attention. The screen slides along the divided scale, 

 and its motion causes the increased overlapping of two sliding 

 wedges of neutral-tint glass. The light is looked at directly 

 through a hole in the screen, and the latter moved along on 

 the scale until the light just ceases to be visible. Dr. Norris, 

 of Birmingham, exhibited experiments showing that the state- 

 ment that India rubber contracts by heat is incorrect. This 

 substance, it is true, contracts in the direction of its length, 



