340 REPORT — 1851. 



electrometers are here omitted as not being absolutely necessary for the ob- 

 jects contemplated ; but they can be, at any time, easily added. 



A sheet of tin-foil is placed under O and P, and in good conducting com- 

 munication with c'. 



S is the Henley electrometer, as modified by Volta (and formerly de- 

 scribed). 



T, the discharger (or spark-measurer), remains as also formerly described. 



V is a rod and balls, adjustable to height, and attached to one of the balls 

 K, in the manner in which k- is attached to the other ball K. 



The distinguisher, used for ascertaining the kind of charge possessed by 

 the conductor, may be either the small Leyden-jar, combined with a gold- 

 leaf electroscope formerly described ; or the electroscope at fig. 2, which 

 retains its charge very long whilst under the glass bell, and may be used for 

 the above purpose without being removed from it*. 



External Apparatus for Insulation, 8^c. — The ap]iaratus represented by 

 fig. 4, divested of the conductor (D), formed part of an instrument formerly 

 alluded to (Report for 1 844), and which was called a pluvio-electrometer. 

 In lieu of the conductor a large copper dish was fixed upon the glass pillar 

 (F), mounted on the tripod which was placed on the leads of the Observatory, 

 and the dish was connected by a wire with a separate insulating apparatus 

 in the Dome, for the purpose of examining the electricity of rain. Deeming 

 the employment of a dish objectionable, however, I made little use of the 

 instrument for that purpose ; but it has been found very convenient in the 

 form here described as an external and portable apparatus, placed in situations 

 where it would have been inconvenient and expensive to have erected a 

 round-house observatory (as fig. 3). Some electrical observers would pro- 

 bably derive advantage from its use, in preference to the rod projecting 

 from a window, as in Volta's, Cavallo's, my own former, and many other 

 experiments ; for it requires a very long rod thus used to get rid of the 

 baneful eflfect of the house-top, chimneys, &c., particularly when the wind 

 passes over the house from the side opposite to that from which the rod 

 projects. 



G (the table) is, in this case, supported by 3 triangular strong boards or 

 frames, attached, by strong hinges, to its under side, and provided with edge- 

 pieces or " feather-boards," for diminishing vibration, &c. ; they can be 

 folded inward, for the sake of portability, &c. 



§■' g' are 2 of 3 pieces of sail-cloth, which, by means of studs (or buttons) 

 on the boards or legs, and of corresponding hooks upon g-' g\ can be laced 

 (with small cords) upon the legs, so as to fill up any or all the spaces between 

 them, and thus prevent the interference of wind with the lamp. 



D, the conductor, and 



F, the glass pillar, &c., are nearly similar to those of fig. 3, but stronger. 

 The warming lamp is much more powerful than the lamp of fig. 3, in order 

 that it may counteract the colder atmosphere to which this apparatus is 

 usually exposed. 



a\ the cylinder, shown by dotted lines, and corresponding in some respects 

 with a\ fig. 3, is of copper, and protects F from rain, &c. 



This apparatus may be placed on the roof of a house, and a wire, or (better) 

 a rod, led from it, may communicate with an internal apparatus as the part 



* It may be convenient to attach this (with its glass bell) to one end of a lon$ arm or 

 bracket revolvable horizontally upon a strong pivot at the other end. 



