V. CLOCKS AND SUNDIALS. 359 



IV. APPARATUS FOR DETERMINING THE EARTH'S 

 MOTION AND DENSITY, &c. 



1841. Apparatus used by Baily in repeating the Ca- 

 vendish Experiment. Royal Astronomical Society. 



Of this apparatus several parts were missing, but have been lately restored. 

 The original portions are the long mahogany box with glazed ends for the 

 torsion balance, and upright column in the middle for the suspension wires, 

 and a box containing three small leaden, one brass, and two ivory balls, two 

 brass cylinde rs, and one leaden lenticular weight. A full description of the 

 apparatus will be found in the Memoirs of the Royal Astronomical Society. 

 Vol. XIV. 



1842. Gauss's Pendulum for demonstrating the rotation of 

 the earth, executed in the year 1853 by Dr. Meyerstein. 



Geodetic Institute of the Observatory at Gottingen {Prof. 

 Dr. Schering}. 



V. ASTRONOMICAL CLOCKS AND SUNDIALS. 



1843. Astronomical Clock, with Sir Gr. B. Airy's Baro- 

 metric Compensation. E. Dent $ Co. 



This clock has been fitted up with a Graham escapement ; it is in other 

 respects almost a counterpart of the new standard clock of the Eoyal Obser- 

 vatory, Greenwich. 



It is found that the tendency of a clock is to gain with a high barometer, 

 and lose with a low one. Compensation is effected in this way : there is a 

 lever, one arm of which carries a float resting on the surface of the mercury 

 in the cistern of a barometer tube ; the other arm carries a horse-shoe magnet 

 which faces the opposite poles of two bar magnets fastened to the pendulum 

 bob. When the barometer rises the mercury in the cistern is depressed, so 

 that the arm of the lever carrying the float falls whilst the other arm rises, 

 thus bringing the horse-shoe magaet closer to the bar magnets ; when the 

 barometer falls the same action takes place in the opposite direction, thus 

 increasing or diminishing a force acting in the same direction as gravity. 



1843 a. Three different Forms of Dipleidoscope. 



E. Dent and Co. 



1. The simple form consists of two mirrors placed at an angle of about 60, 

 and in front of them a plain unsilvered glass, the whole combination being 

 mounted, for the sake of conveniently taking observations, in a small cast 

 metal pyramid. The optical arrangement operates in this way. Kays from 

 the sun fall upon the front glass, and part are reflected from it and from an 

 image ; but the remaining part pass on, and meeting first one and then the 

 other mirror, are reflected back through the front glass, and form a second 

 image. The instrument is to be placed so that these images shall appear 

 together in the field of view, a minute or two before apparent noon. Then 

 what is seen is this : as the sun advances to the meridian the two images will 

 approach, they will touch,. and gradually cover one another (this observation 

 gives the instant of apparent noon) ; they will continue to move on, and will 

 finally leave one another ; and each of the observations of contact, superim- 

 position, and parting contact will be each separately and together available for 

 determining true time. The base-plate which accompanies the instrument, it 



