Intelligence and Miscellaneous Articles. 79 



Bagshot, and the occurrence of fossils in the Upper Bagshot of that 

 district was recorded. 



The conclusions that the authors came to were, that a well-marked 

 pebble-bed was almost always present, marking the division between 

 the Upper and Middle Bagshots, but that there were other pebble- 

 beds of a less persistent character occurring, both in the Middle and 

 Lower Bagshot ; that the Lower Bagshots generally consist of false- 

 bedded sands with clay laminae and no fossils except wood, whereas 

 the Upper Bagshots are rarely false-bedded, and are characterized by 

 the absence of clay bands and the presence of marine fossils ; and 

 that the Middle Bagshot is a well-marked series consisting of green 

 sands and clays. 



They claimed, in conclusion, that there was no reason for disturb- 

 ing the old reading of the district, and that there was no evidence of 

 an overlap of the Lower Bagshots by the Upper. 



X. Intelligence and Miscellaneous Articles. 



ABSOLUTE SPHERICAL ELECTROMETER. BY M. LIPPMANN. 

 ^HIS instrument consists essentially of an insulated metal sphere 

 -*- which is raised to the potential we desire to know. This sphere 

 is constructed so as to divide into two hemispheres, which are 

 movable in respect of each other, and which repel with a force 

 equal to / when their system is electrified. 



It can easily be shown that f and V are in the very simple ratio 



In order to have V it is sufficient to measure /. This measure- 

 ment might be made by several methods ; I have adopted the 

 following : — 



In the first place, if the apparatus intended to measure / were 

 external to the metal sphere, we should be obliged to put it so far 

 off that its action had no disturbing influence on the distribution 

 of the electricity. I have accordingly preferred to put the whole 

 inside the electrified sphere itself, which is hollow. 



One of the hemispheres is fixed ; the other, which is movable, 

 is suspended by a trifilar system, that is to say composed of three 

 vertical wires of equal lengths. When repulsion is produced, the 

 movable hemisphere can only be displaced parallel to itself ; the 

 three wires make then a small angle with their original vertical 

 position ; a is measured by a method of reflection by means of a 

 mirror fixed to two of the wires *, and seen through a small aperture. 

 It will be seen that if p is the weight of the movable hemisphere, we 

 have 



f—p tan q ; 

 and therefore 



p tan a = JV 2 . 



Hence it is sufficient to know the weight p which is fixed ; the 

 value of the radius of the sphere is immaterial. 



* The apparatus was constructed by MM. Breguet. 



