338 REPORT — 1851. 



the zero-point being opposite to the line where the straws nearly touch each 

 other when unelectrified. 



cO is a small brass tube or cap fitted upon the cap of a'*, but whose interior 

 cylindrical surface stands at the distance of about ^'Ca of an inch from the 

 coating of lac on a* ; it* can be removed at pleasure*. 



B is an electrometer, similar to A in all respects, excepting as regards its 

 pair of straws 6^, which are rendered so much heavier than the straws of A, 

 by filling their cavities with the prolonged wire of their hooks, as to diverge 

 (when both are equally electrified) exactly ^th as much. This arrangement 

 agrees with Volta's prescription for his electrometer No. 2, and the exact 

 accordance of the two instruments is ascertained by his halving process^f. 



C is the conductor, consisting of a very light conical tube of copper about 

 3 feet 6 inches long, and furnished with a brass cap below, which screws 

 upon the cap of a*. 



c' (fig. P) is a helix of small copper wire, the lower and smaller part of 

 which fits upon the upper extremity of C ; the upper part being considerably 

 larger. 



c^, a solfanello (of Volta), i. e. a sulphur candle, composed of about 10 

 threads of lamp-cotton coated and imbued with sulphur whilst in a melted 

 state. It is placed in the larger part of c'. 



D is a mahogany case placed upon a portable staft'or table, or a post. 



d' d', &c. grooved pieces, between which the lower parts of A and B may 

 be slid, and the instruments be thus properly packed for transport. 



d^, a little drawer containing a supply of solfanelli, &c. 



A hollow walking-cane may contain C, disjointed for the sake of porta- 

 bility ; and tubes might be occasionally slid through the bottoms of a' and i' 

 to embrace a^ and 6\ thus preventing their violent vibration, &c. in transport. 



When these instruments are to be used in Volta's or Bennet's manner, c' 

 may be shd upon C, and c- (lighted) into c' ; when in Saussure's manner, a 

 pointed wire may be substituted for c' and c- ; when in Cavallo's, Erman's, 

 or Peltier's (inductive) manner, a ball may be attached to the end of C, or C 

 may be disjointed and a ball attached to its lower joint ; or the instrument 

 may be used without any such conductor, as may be most suitable to cir- 

 cumstances of locality, &c. 



In all cases where the atmospheric charge is not extremely minute, straws 

 are infinitely preferable to gold leaves, which can never be safely transported 

 in their instrument. 



The Peltier Electrometer, or (rather) Erman's, which has been particularly 

 described by both inventors, can scarcely be said to be in working order, its 

 insulating capacity having greatly diminished. 



Electrical Observatory for transport. — The expense and difficulty of con- 

 veying to distant stations apparatus for electrical observations so large and 

 heavy as is ours, have furnished the motives for arranging some instruments, 

 already existing at Kew in the form represented by fig.3, PlateXVIII., which 

 shows that a Very efficient apparatus may be constructed at small compa- 

 rative expense, and presenting considerable facility of transport. 



The round-house AA is drawn in half-section ; its proportions are derived 

 from a square portable house which came to Kew from Woolwich for some 



* Since the above was written, an electrometer of tliis kind has been presented to Lieut. 

 Cheyne for employment in Sir Edward Belcher's north polar expedition, 

 t Vide Opera del Volta, tome i. parte ii. p. 13 et seg. 



