458 Mr. R. L. J. Ellery on a Pendulum- Ekctrograph. [June 18, 



XI. " Description of a Pendulum-Electrograpli now in use at the Mel- 

 bourne Observatory." By R. L. J. Ellery, Government Astro- 

 nomer to tbe Colony of Victoria. Communicated by Balfour 

 Stewart, Esq., F.R.S. Received May 23, 1868. 



(Plate II.) 



About eighteen months since, being desirous of making some improve- 

 ments in our mode of observing atmospheric electricity, I tried a series 

 of experiments with Sir William Thomson's various methods of obtaining 

 observations and measures of the electric state of the atmosphere; the re- 

 sults of these were so very satisfactory that I had a divided-ring reflecting 

 electrometer made, as nearly similar as possible, judging from the descrip- 

 tions available, to those used by himself. This was attached to a water- 

 dropping collector, and I obtained the deflections of the needle measured by 

 means of a telescope and reflected scale. Prior to this, all observations of 

 atmospheric electricity were made with Quetelet's modification of the Pel- 

 tier electrometer, where the needle and its little directing magnet are sus- 

 pended by a cocoon fibre instead of on a point. The use of this, however, 

 was very troublesome, involving its being carried to the highest part of the 

 building at every observation, brought down, and placed on its stand within 

 doors, the needle brought to rest by a magnet, and, after reading off the 

 force of repulsion, the ascertaining of the character of the electricity by a 

 separate operation. After using the divided-ring electrometer for a few 

 weeks, it became apparent that no method of observing atmospheric elec- 

 tricity that was not continuous could possibly afford results that would 

 embrace the numerous and rapid changes which take place. I found also 

 in observing with the divided-ring electrometer that the torsion of the pla- 

 tinum wire was uncertain, requiring very frequent alteration of the zero- 

 point, especially after great disturbances. 



I therefore endeavoured to devise an electrograph that should act by 

 gravity instead of torsion ; and for this purpose the electrometers of Sir 

 AV. Thomson and the photographic registration method adopted in the Kew 

 magnetographs afforded me a sufficient groundwork. In August last I so far 

 succeeded as to obtain regular photographic curves of the electric condition 

 of the air about 18 feet from the earth's surface; experience of the first 

 temporary apparatus suggested modifications ; and in November an im- 

 proved instrument was erected, of which a brief description was read before 

 the Royal Society of Victoria in December. Some defects in the perform- 

 ance of this, however, led to even a further modification ; and since Decem- 

 ber last the improved electrograph has performed most satisfactorily. 



Plate II. will explain its construction. It may be generally described 

 as consisting of — 



1 . Reservoir of electricity. 



2. The pendulum. 



3. Electrodes. 



