III. SURVEYING. 739 



The air pump served to exhaust the receiver, the vibrations being made 

 in a vacuum. 



The coincidences were observed by the small telescope erected at a 

 distance. 



3134. Pendulum Apparatus. Sabine. 



Kcw Committee of the Royal Society 



Pendulum vibration apparatus used by Sir E. Sabine, and similar to that 

 now in use by the Arctic Expedition under Captain Kares, R.N. 



It consists of a massive iron triangular frame, supporting a pair of agate 

 planes, upon which the invariable pendulum is swung ; a strong wooden 

 frame, carrying a Sheltou clock, and a telescope, with which the coincidences 

 of beat of the detached pendulum and that of the clock are observed. 



3133. Photograph of a Be version Pendulum Appa- 

 ratus, distance of knife-edges 1 metre from each other, executed 

 for the Central Bureau of the European Measurement of Degrees, 

 Berlin. A. Repsold'and Sons, Hamburg. 



The apparatus consists of the stand made of brass tubing, the pendulum, 

 the scale, and the comparator. The pendulum is a brass tube provided with 

 knife edges. The scale is of the same tube as the pendulum, and contains a 

 steel tube, and this again a zinc tube, both of which are united with each 

 other and with the brass tube in such a manner that for the determination 

 of the temperature a small divided scale which moves by the different ex- 

 pansion of steel and zinc, may be read off near one of the end divisions of the 

 main (brass) tube. The divided surfaces are in the middle line of the tube. 

 The scale may stand with either end upwards. By rotation of the comparator 

 round its foot the two microscopes may be alternately directed upon the scale 

 and upon the suspended pendulum. 



3100. Photograph of a Base-measuring Apparatus, 



in course of construction for the United States Lake Survey, 

 Detroit. A. Repscld and Sons, Hamburg. 



A measuring rod 4 metres in length is used, consisting of a steel and 

 a zinc rod, enclosed together between rollers in a wide iron tube, and 

 with divisions upon platinum at the ends. The measuring rod rests on 

 its extremities (below the divisions) upon its supports, by \vhich the ad- 

 vantage is attained that each support remains in place for tAvo positions, 

 and, therefore, only one has to be arranged for each new position. Before 

 making a measurement microscopes are placed vertically upon separate 

 supports above the rod-supports, and beneath these the rod is carried forward. 

 To fix the daily termination a cylinder with a level is arranged perpendicu- 

 larly under the last microscope on the small hemisphere of a terminal plate on 

 the grouud, and by the microscope a division on the upper terminal surface 

 of the cylinder is read off in the two positions differing by 180. The appa- 

 ratus is accompanied by a comparator for the comparison (1) of the measuring 

 rod with a standard metre ; (2) of the measuring rod with a spare measuring 

 rod. In the first case the standard metre is laid, together with the measuring 

 rods, upon a' carriage 4 metres in length, and compared one by one with the 

 intervals of the measuring rod divided into metres. For this purpose the 

 carriage is moved to and fro upon the base piece under two microscopes 

 placed upon a common bearer. This bearer may be pushed along the whole 

 length upon the base piece (like the standard metre upon the carriage). In 

 the second case the two measuring rods are laid side by side upon the 

 carriage, and compared under the microscopes, one of which is attached to 



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