3^8 DORP AT AND POULKOVA. 



The reduction of tlie "eye and ear" observ-ations having shown that the 

 Houth-Wetzer clock in the west room, and the Kessel's normal clock originally 

 placed in the central rotunda, were aflected by the unavoidable changes of temper- 

 ature, the former was replaced in 1861 by a dial connected by electricity with tlie 

 normal clock, thus avoiding the laborious comparisons by chronometer that had 

 been until that time earned on daily. In order to seciu'e a still more uniform 

 temperature the normal clock was then placed in an inner vault underneath the 

 rotunda, where the daily thennometric term in the clock rate is quite impercep- 

 tible. Since 18G2 the obsei-vations have been recorded upon a Krille's chrono- 

 graph, which stands in a warmed room adjacent to the observing room, and ditlers 

 from those in common use in America principally in that the observei-'s pen is 

 independent of the neighboring clock pen, and in having a very convenient 

 arrangement by which the observer at the transit can at will stop the revolving 

 cylinder or set it in motion again. The clock automatic circuit-breaker is that 

 of Krille. It consists of a thin vertical slip of mica at the extremity of a short 

 arm attached at right angles to the uppex portion of the pendulum, and in tho])lane 

 of vibration ; at every second the mica cuts through a sms.ll horizontal thread 

 of mercury through which the electric current is passing. The Muston mean time 

 clock was in 18G6 connected with the central telegraph station in St. Peters- 

 biu'g, and regulates several sympathetic clocks. A noonday signal is also auto- 

 matically given. 



The exquisite small Meridian Traiisif, made by Brauer, and now found in an 

 appropriate building southwest of the larger observatory, was used in the longi- 

 tude expeditions to Dorpat, Moscow, &c., and has been employed by Fritsche 

 in the series of lunar observ^ations recently published by him in the Bulletin of 

 the St. Petersburg Academy. A mate to this fine instrument is to be found at 

 the naval observatory at Cronstadt. The five-inch Steinheil objective, mounted 

 in 18G6 parallactically in the east dome of the small auxiliary observatory erected 

 in 1863, 100 yards south of the principal one, is intended to be used by 

 "Wagner in an investigation into the relutive parallaxes of some of the brighter 

 stai's. The evidences of the extraordinary accuracy attained with the Meridian 

 Transit are such as to ju&tify the expectation that very decisive results will ensue 

 from this renewal of the method so lately applied by Auwers to the determina- 

 tion of the parallax of 34 Groombridge. 



The celestial photometry which has remained until lately in so crude a state, 

 thanks to the labors of Steinheil, Sei»lel, and Zijllner, promises in futiu'e to rank 

 as an exact science. An ingenious Photometer^ invented and made by Professor 

 Schwerd, of Speyer, was mounted in 1866 by Messrs. Smysslotf and Berg in the 

 west dome of the anxilliary observatory, and oH'crs a fine opportunity for research 

 in a field that has as yet been but too little cultivated. A mate to this unique 

 instrument has been ordered for the observatory of Bonn by Professor Arge- 

 lander, who has signified his intention of devoting his future years to its use. 



A little to the northeast of the central building is tlie convenient and tasteful 

 observatory erected at the expense of the military academy, and furnished with 

 a clock, a fixed transit, and very many portable instruments, together with very 

 convenient arrangements for their use. This structure, completed in 1857, is of 

 course exclusively for the use of the officers of the geodetic division of the Kicolas 

 Military Academy whilst pursuing at Poulkova their course in- practical astron- 

 omy. Among the works executed by these oflicers under Dollens's directions are 

 several whose results may be looked for with general interest, such as the twelve 

 repetitions of the measurement of a short base line, in the year 1865, and the 

 observ^ations made in 1866 for the investigation of the local attraction of the 

 plumb line in the neighborhood of Poulkova. 



In recounting the larger fixed instruments of the Central Observatory, we must 

 not omit a few words ct)ncerning the smaller portable ones, of which the insti- 

 tution possesses several fine specimens of the best workmanship of Ertel, the 



