Apparatus for Ek-Brkal Expenmenis, 435 



that might happen by its ftriking any metallic fubftance which is not continued to the 

 earth. 



But buildings furnlflied with fuch condudors have been ftruck by lightning : it is obvious,, 

 therefore, that the lightning muft either have pafTed to the damaged part without approach- 

 ing the conduftor, or it muft have ftruck from the condudor to the damaged part : for, al- 

 though all metallic fubftances belonging to a building may be placed at a diftance from the 

 conduaor, yet it cannot be fuppofed that any diftance within the limits of a building can 

 exceed ihc Jinking dijlaitce of lightning. 



The apparatus with which this experiment was performed, confifts of an eleftrlcal ma- 

 chine having a glafs cylinder of about 18 inches diameter, confttutled by Mr. Nairne, in 

 the form of his patent electrical machines. 



Two hollow cylinders *, made of thin wood covered within and without with tinfoil, 

 are placed, in a vertical pofition, upon two infulated circular boards ; the glafs pillars which 

 fupport the infulated boards are about a foot in height, and are fixed into thick circular 

 boards, covered alfo with tinfoil, and refting upon the floor of the room. 



The interior diameter of each of thefe cylinders is 18 inches; the height from the infu- 

 lated board upon which it ftands, eight feet fix inches, including an hemifpherical top ; and 

 the diftance between them is about five feet. 



From the centre of each of the boards, that reft upon the floor, rifes athlck glafs pillar, 

 which pafles through a hole of a foot diameter in the centre of the infulated board into the 

 hollow of the cylinder. This pillar of glafs extends about fix inches above the infulated 

 board, and fupports, in a vertical pofition, another cylinder made of wood, and covered with 

 tinfoil. 



This Interior cylinder is one foot in diameter, and its height is fix feet fix inches, Includ- 

 ing an hemifpherical top. It Is placed exadVly parallel to the exterior cylinder, the interval 

 in every part being three Inches. To this Interior cylinder is fixed a metallic chain, by which 

 it can he conne£led with the exterior cylinder, fo as to form only one infulated body covered 

 with tinfoil ; or can be connedled with the earth, whilft the exterior cylinder remains Infu- 

 lated. 



The apparatus has alfo a moveable Infulated body, made of cork covered with tinfoil, to 

 leprefent a cloud moving in the air. 



And alfo two metallic rods placed upon infulated ftands, that they may be connefted with 

 or detached from, the earth at pleafure. Thefe rods are ufed to reprefent the conduftors 

 or any metallic fubftances that belong to buildings ; their forms are varied t)y fixin ' balls or 

 points upon the'ir upper extremities ; and the ftand in which they are placed being hollow, 

 they can be placed at any height by means of a fcrew. 



To fliew the eflieds of the pafl"age of the eleftrical fluid, two Infulated metallic halls are 

 placed within thcjlriking dijlatice of each other ; and, in the interval between them, is put 

 fome inflammable fubftance, fuch as dried powdered rofin mixed with cotton. 



* It may be of advaotage to the reader to confult the literal refereiites ut the end of this mctnoir before he 

 pioccedi. N. 



3K2 EXP El 



