ASTRONOMICAL AND GEODETIC OBSERVATIONS. 



29 



PENDULUM EXPERIMENTS. 



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The pendulum observations were made for the purpose of ascertaming the relative 

 force of gravity at Cambridge, Massachusetts, and at the winter 

 quarters of the expedition in North Greenland. The pendulum 

 was expressly made for the occasion by Bond & Son, Boston. It is 

 an invariable, reversible, brass pendulum, perfectly symmetrical in 

 all its parts, as shown in the annexed figure. It is very nearly 

 synchronous, though not convertible, as its form at once indicates. 

 Its total length is 5 feet 7.f inches, width 1.4, and thickness 0.7 

 inches ; distance between the knife-edges 39.4 inches. The steel 

 knife-edges are 14.2 inches from the ends of the bar, 3 inches 

 long, 0.3 inches high, and 0.27 inches Avide at the base; their 

 section is triangular. The weight is 21.92 pounds, hence its 

 specific gravity 8| nearly. The knife-edge, which runs through a 

 perforation of the bar, rests upon steel plates. They are screwed 

 to a brass plate, and supported by a heavy block of wood, which is 

 fastened to the case in which the pendulum swings. There is no 

 adjustment for horizontality of the supporting steel plates other 

 than what is given by the vertical position of the case. The arc 

 of vibration is read off on a scale at the bottom of the case, which 

 has a glass door in front permitting a view of the whole pendulum. 

 Two thermometers are permanently fastened inside the box, one 

 just above the support, the other on a level Avith the swinging 

 knife-edge. 



There is a preliminary reduction of the observations at both 

 stations by Mr. Sonntag ; the present independent reduction differs 

 from it by a more complete and critical use of the materials ; no 

 attempt, however, of combining the resulting number of vibrations 

 at the two stations had been made by Mr. Sonntag. 



The following explanatory note is extracted from the record of 

 the experiments at the Harvard College Observatory : — ^ "^ 



. " Pendulum suspended in transit room of Observatory of Har- 

 vard College, Cambridge, and its vibrations observed by G. P. 

 Bond, Director, and T. H. Safford, Assistant." 



In the following pages are the times read off from the record 

 sheet of the electric register. The signals always commence with 

 the transit of a mark on the pendulum from right to left, seen in 

 the telescope (which does not invert). Different marks were used 

 for different sets,^ but the same mark was always observed both 

 right (R.) and left (L.). 



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' Owing to defective illumination tlie point first selected, which was the knife-edge, could not 

 always be seen, and others were taken — all of them near the axis. 



