402 F. Dellmanu tni Atmospheric Electricity. 



immediately surrounding air^ but are vitiated by the action of 

 the conductor leading to the measuring instrument. 



It is Avell known that it is only since 1844 in Brussels and 

 Kew, and for the last few years in Munich, that observations on 

 atmospheric electricity worthy of the present state of science 

 have been instituted. The Brussels observations are imperfect 

 from being made only once a day, and those at Kew from being 

 made with imperfect instruments. At Brussels the Munich obser- 

 vations are considered, not unjustly, to be deteriorated by faulty 

 graduation of the measuring instrument ; but this objection ap- 

 plies more or less to the Brussels observations themselves. In 

 undertaking similar observations for the Prussian government, 

 my endeavour was to avoid the errors of the existing observa- 

 tions, and to appropriate as much as possible their good qualities. 

 At all events the method followed at Brussels appeared to me to be 

 the best, though it could only be partially applied, since I wished 

 to make use of my own instrument*, which undoubtedly excels 

 all others in precision and convenience. All difficulties arising 

 from the absence of a tower, and from the disturbing action of 

 two neighbouring and much higher houses, w^ere overcome by 

 the following construction of the collecting apparatus. 



I. Appakatus. 



a. The Collector. 



Two strong iron bars, a and b (see Woodcut), at a distance of 

 about 2metresfronieachother, were passed through the S.W.gable- 

 wall, and in the inside fastened to wood. The upper bar a is about 

 1| metre distant from the summit of the gable. Each bar projects 

 about a foot from the wall outside, and bears a ring at its extremity, 

 which can be easily withdrawn and replaced by others. Through 

 these rings passes a deal rod c, 22 feet long, capable of being 

 drawn up and down by means of a rope passing round a drum 

 to which an iron ratchet-wheel is attached. The deal rod when 

 drawn up is held fast by means of a ratchet. In order that the 

 rod may be always drawn up to the same height, the hook d at 

 its lower extremity, to which the rope e is attached, is bent at 

 right angles, after being screwed into the axis of the rod, so that 

 the bent part tits close to the under surface of the rod, and by 

 striking against the lower ring, arrests all further ascent. The 

 rod is placed between two windows,/ and g, before one of which 

 is a balcony, whilst before the other hangs a brass wire h, pro- 

 vided at the bottom with a brass handle i, and at the top fastened 

 to a copper crooked lever {JVinkelhebel) k, which passes through 

 the deal rod about 6 inches from its upper end. The upwards- 



* Described in PoggendorfF's Aimalen, vol. Ixxxvi. p. 524. 



