80 Intelligence and Miscellaneous Articles. 



re bain the vibration-number, we gain advantage from the shortness 

 of the fork, and hence from the increase of angular motion of the 

 prongs. 



It was suggested to me that possibly bell-metal had the property 

 of accepting motion more readily than steel. To test this point I 

 made a pair of Ut 3 steel forks, shorter than Kcenig's, and of course 

 thinner, in order to retain the vibration-number. These forks 

 behaved just like the bell-metal forks. Further, I made a pair of 

 Ut 4 forks as long as Kcenig's Ut 3 forks, and of course thicker. 

 These behaved like Kcenig's Ut 3 forks. Finally, taking a Kcenig 

 Ut 3 fork on its case, and one of the short Ut 3 forks also on its 

 case; on placing them twenty feet apart, it was found that, on 

 exciting Kcenig's fork, my short fork responded well, whereas on 

 exciting the short fork Kcenig's did not respond at all. — Silliman's 

 American Journal, December 1876. 



NOTE ON THE CORRECTION OF THE VARIATIONS OF RATE OF 



ASTRONOMIC PENDULUMS PROCEEDING FROM DIFFERENCES OF 



ATMOSPHERIC PRESSURE. BY A. REDIER. 



Experiments, carefully made, at Brussels, Berlin, and London 

 have proved that a seconds-pendulum varies 0-6 of a second per 

 day between the two extreme barometric pressures. A pendulum 

 perfectly regulated to 730 millims. pressure, would lose 0*6 second 

 at 790 millims. 



Mr. Airy has had constructed by Mr. Dent the first pendulum 

 known which corrects these errors. In this construction a horse- 

 shoe magnet, brought more or less near to an armature attached 

 to the bob accelerates or retards the motion of the pendulum. The 

 shifting of the magnet is effected by means of a float placed upon 

 the siphon of a mercury-barometer. The result is perfect ; but, 

 in presence of the difficulties attendant upon the execution of this 

 method, it appeared to me useful to seek out a more simple one. 



I have tried, with some success, the effect of the friction of the 

 air on the surface of the bob. An enclosed bob does not move at 

 the same rate as one in free air ; and I can with ease automatically 

 augment or diminish the free space, according to the atmospheric 

 circumstances ; but this procedure, as well as Mr. Airy's, is again 

 too complicated. There is a simpler and surer means of arriving 

 at the result sought. 



Suppose attached horizontally to the top of the bob the barome- 

 tric case of an aneroid. The bottom of the case thus fixed faces 

 the suspension. Upon the opposite flexible wall, at its centre, is 

 sealed a mass, which rises or falls like the barometric case. "When 

 the atmospheric pressure increases, the mass approaches towards 

 the suspension of the pendulum ; and this goes faster, thus com- 

 pensating the effect itself of that pres'sure. If the pressure dimi- 

 nishes, the reverse effect is produced. The motions of the case 

 being known, it only remains to calculate the value of the movable 

 mass as a function of the weight of the bob and of the position of 

 the mass in relation to the bob. — Comjrtes JRendus de V Academic 

 des /Sciences, Dec. 13, 1870, p. 1174. 



