ISfi 



ELECTRICITY. 



TADLI II. Electrical State of the Almotphtrt in 1791. 



Bead'i ex- 

 periment* 



r>ev 



Rcad' ex- 

 ptrinifOla, 



General re- By comparing this Table with the preceding, it will 

 tultt. appear that the differences in the electrical state of the 



atmosphere have been very considerable. Positive elec- 

 tricity appeared only 2tl times in 1790, whereas it ap- 

 peared 4'.J:> times in 1?9I. The number of times in which 

 negative electricity appeared was nearly equal, being 

 156 times in 17!X>, mid 157 times in 1791. The num- 

 ber oi days in whirl) sparks were drawn was 98 in 

 1790, and 106 in 1791. Mr Head, however, states, 

 that the greater number of times in which positive elec- 

 tricity appeared in 1791, was partly owing to his having 

 employed a hotter apparatus. 



Mr Read has drawn the following conclusions from 

 his numerous experiments on atmospherical electricity. 



1. In a course of moderate weather, the electricity 

 of the atnosphere is invariably positive, and exhibits a 

 flux and reflux, in which it undergoes an increase and 

 a decrease twice in every 24 hours. 



2. The electricity is strongest about 2 or S hours af- 

 ter the rising of the sun, and some time before and af- 

 ter sunset ; and it is generally weakest between noon 

 and 4 o'clock. 



S. The periodical electricity of the atmosphere seems 

 to be manifestly influenced by heat and cold. Hence, it 

 is obvious, why warm small rain is weakly electrified, 

 while cold rain which falls in large drops is the most 

 intensely electrified of any. 



Exjwn- The most recent experiments on the electricity of 



menu of Mr the atmosphere have been made by Andrew Crosse, 

 Esq. of Broomfield, near Taunton, by a very exten- 

 sive atmospherical conductor, consisting of an insu- 

 lated copper wire, V^th of an inch thick, extend- 

 ed lictween two vertical masts from 100 to 1 1O feet 

 high. No exertion was spared to render this appa- 

 ratus the most extensive and perfect that has been 

 contracted. The insulated wire was made no less 

 than one mile and a quarter in length, but having been 

 exposed to depre.lationx, and liable to injury frmu other 

 causes, it was shortened to 1800 feet. Every contri- 

 vance has been tried to insulate this wire, but Mr 

 Crosse has not yet been able to preserve the insulation 

 during a dense fog or a driving snow. A contrivance 

 was adopted to lower the insulators, for the purpose of 

 cleansing them from spiders' webs ; and it was necessa- 

 ry to fix the wire very securely, in order that it might 



be nble to resist the weight of innumerable swallows 

 that often perched upon it. and ol' wood pigeons ;:nd , '--01 Mr 

 owls that often flew against it with considerable t'oicc. 

 This apparatus has been in use more than eighteen 

 months, and has enabled Mr Cros-e to draw the follow- 

 ing important conclusions, many of which confirm the 

 observations of preceding authors. 



1st, The clc. tricity i* invai iaiily po<i:i\e in the or- 

 dinary state of the atmosphere. It U always weakest 

 during the night. It increases at sunri.-r, then de- 

 crease* towards the middle of the day ; and regaining 

 its strength as the sun decline", it again dimini- 

 and continues feeble during the night. 



2d, Fopts, rain, snow. i.. '1. ;.nd sleet, produce changes 

 in the electrical state of the wire. '1 he electricity is 

 negative when they first ;')-.pcar. It frequently i-h.mgc* 

 to positive, gradually increasing in strength ; and then 

 decreasing in a similar manner, and changing from po- 

 sitive to negative every three or four minutes. Those 

 phenomena have been so constantly observed, that 

 whenever the wire appears m-gatively electrified, it is 

 considered as a certain indication, that either ram. snow, 

 hail, mist, or a thunder cloud, is in the neighbourhood. 



.'!d, The approach of a charged cloud at first some- 

 times produces positive and -"UK-times negative elec- 

 tricity ; but whatever be the kind of the electricity 

 which first appear-., its intensity increases to a certain 

 degree, and then diminishes and disappears, and is 

 succeeded by the opjtoMtc electricity ; which increases 

 to a higher degree than the first had done, and then 

 diminishes, and vanishes, and is again succeeded by 

 the electricity which first appeared. These varying 

 alternations of positive and negative electricity are of- 

 ten very numerous, and on different occasion* sneered 

 one another with different degrees of rapidity. The 

 electricity, in general, l>ccn.u-s more intense at every 

 repetition, till a copi' dense stream of sparks 



i'nint the atmo plu-rical conductor to the recei- 

 ving ball, stopping at intervals, and returning with re- 

 doubled energy. Under these circ; . a strong 

 current of air proceed* from the wire, and the appara- 

 tus connected with it. Whenever the lightning flashes, 

 an explosive stream, attended with a peculiar noise, 

 passe-, between the balls of the apparatus, and throws 

 a brilliant light on every surrounding object. When 



