174 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



observed and illustrated. A bulb, three inches in diameter, 

 is blown at the end of a glass tube eighteen inches long. In 

 this a fine glass stem, with a sphere or disk of pith at each 

 end, is suspended by means of a fibre of silk. The bulb is 

 then perfectly exhausted and hermetically sealed. Instead 

 of pith, disks may be made of iron, metal, cork, or other sub- 

 stances. The apparatus, when constructed with proper pre- 

 cautions, is so sensitive to heat that a touch of the finger on 

 a part of the globe near one of the disks of pith will drive 

 the index around over a quarter of a revolution, while it fol- 

 lows a piece of ice, as the needle follows the magnet. With 

 a large bulb very well exhausted, a somewhat striking effect 

 is produced. When a lighted candle is placed about two 

 inches from the globe, the glass stem with its pith disks os- 

 cillates to and fro through gradually increasing arcs, until 

 several complete revolutions are made, when the torsion 

 of the suspended fibre offers a resistance to the revolutions, 

 and the bar commences to turn in an opposite direction. 

 This movement is kept up with great energy and regularity, 

 like the movements of the balance wheel of a watch, as long 

 as the candle burns. A modification of this apparatus, in 

 which a glass thread is substituted for the silk fibre, allows 

 quantitative as well as qualitative observations. The sensi- 

 tiveness of the apparatus to heat rays appears to be greater 

 than that of the ordinary thermo-electric multiplier. Thus 

 the obscure heat rays from copper, at a temperature of 100, 

 after passing through glass, produce a deflection on the scale 

 of 3^ divisions, while under the same circumstances no cur- 

 rent at all is detected in the thermo-pile. Nature, XL, 494. 



rood's application of zollner's horizontal pendulum. 



The paper of Professor O. N". Rood, on the application of 

 the horizontal pendulum to the measurement of minute 

 changes and the diminution of solid bodies, although read in 

 1874, has only recently been published ; and from it we learn 

 the details of the instrument proposed by him as an improve- 

 ment on Zollner's horizontal pendulum. This proposed im- 

 provement consists essentially in an inflexible rod placed 

 horizontally, and supported in that position in mid -air by 

 vertical wires or springs, stretched in such a manner that 

 the influence of gravity on the rod is no longer sensible, 



