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RESULTS OF THE PENDULUM OBSERVATIONS. 



L. C. BERNACCHI, F.R.G.S. 



INTRODUCTORY. 



Among the investigations that the " Discovery " Polar Expedition had placed upon its progi'amme were 

 those for the determination of the force of gravity in a high southern latitude. As there could be no 

 question of anj'thing but relative determinations, it was decided, after consulting Professor \'0N Helmert, 

 to apply for the loan of the Stiickrath pendulum apparatus of the South Kensington Museum. 



Instrumental Equipment. 



The apparatus was lent to the Expedition by the authorities of the South Kensington Museum. The 

 complete outfit comprised a set of three quarter-metre invariable pendulums with agate knife-edges, to 

 swing on three separate agate planes, an air-tight case in which they were swung, a dummy or temperature 

 pendulum, flash apparatus, air pump, dry cells, and various accessories. 



The stand was a heavy metal one, provided with levelling screws, and arrangements for starting, 

 stopping, raising and lowering the pendulums from the outside. A heavy case fitted over the pendulums 

 and rested on a smooth level rim of brass at the base of the stand, which was fitted with a similar rim so 

 that the rims came in close contact, a thin layer of vaseline being previously spread between them. The 

 knife-edges were made horizontal by means of small pendulums with levelling tubes in their heads. Two 

 windows in the case permit the mirrors at the top of the pendulums and the thermometer being seen. 

 The bulb of this thermometer was inserted in the stem of a dummy pendulum of the same size and metal 

 as the swinging ones and held in the case near them. 



The flash apparatus is for the purpose of observing coincidences between a chronometer and the 

 swinging pendulum. An electromagnet in circuit with a break-circuit chronometer moves a shutter at the 

 end of each second, thus throwing a flash of light through a narrow slit. The image of this slit is seen in 

 an observing telescope supplied with a reticle of wires. These are so adjusted that when a pendulum is at 

 rest the image of the slit coincides with one of the wires. When the pendulum is moving, the apparent 

 position of the flash depends on the position of the pendulum when the reflection occurs. The period 

 being slightly greater than half a second, the pendulum falls behind the chronometer at each swing. If s 

 is the number of seconds lietween two coincidences, or the coincidence interval, then the pendulum executes 



2s -I vibrations in -s seconds, hence its period = -j^ — - seconds. 



The Reductions to Standard Conditions. 



The eflfects of changes of temperature and pressure were investigated at the National Physical Laboratory 

 (Kew Observatory Department), and coefficients were deduced. The following are the corrections employed, 

 applying to all three pendulums the mean results obtained : — 



Temperature correction to reduce to 0° C. 



-o-oooooie-t/, 



where t is the observed temperature in degrees Centigrade. 



D 2 



