Art. XIX. — A Description of a New Pendulum Apparatus, 

 with Half-Seconds Pendulums. 



By R. L. J. Ellery, C.M.G., F.R.S., F.R.A.S. 



[Read 14th December, 1893.] 



When the Kater's Invariable Pendulums, lent by the Royal 

 Society of London for the Gravity Survey of Australia initiated 

 by this Society, arrived and were installed at the Observatory, the 

 cumbrous character of the whole apparatus convinced me that 

 the cost of transport and of installation at the various observing 

 stations would be a serious hindrance to the undertaking. When, 

 therefore, some months later, an Austi'ian officer of the warship 

 Saida brought a set of half-seconds pendulums, for making a 

 series of gravity observations at the Observatory for connection 

 with the Vienna base, I was struck with the immense convenience 

 of transport and facility in making the necessary observations 

 which these instruments afforded, and as I soon ascertained that 

 the results appeared in every respect as good as with the larger 

 pendulums, I determined to get a set made to test the question, 

 hoping that they might be found efficient for the survey work. 

 These pendulums and apparatus are now complete, and are on 

 the table for the inspection of members. They are made after 

 the plan adopted by Colonel von Sterneck of Vienna, but with 

 certain modifications, and are in some respects similiar to the 

 half-seconds pendulums recently used in the United States 

 Coast and Geodetic Survey, and described by Mendenhall in his 

 report for 1891. 



The apparatus consists of three half-seconds pendulums, a 

 coincidence or flash apparatus, a pendulum-stand, thermometers, 

 air-pump, &c., as well as a break circuit chronometer. 



In the design and construction of the pendulums, the chief 

 and essential requirement of invariability and symmetry of form 

 have had the first consideration. Great care was also taken as 

 regards the metal of which they were first formed, to secure 



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