234 PHILOSOPHICAL TRANSACTIONS. [aNNO 1751. 



considered. It consisted of a glass tube nearly 3 feet in length, and of almost 

 3 inches in diameter. A ring of brass, exactly fitting this tube, was cemented 

 to both its extremities, into each of which was screwed a hollow brass cap, nearly 

 of an hemispherical figure. Into the top of one of these caps was adapted a brass 

 box of oiled leathers, through which was admitted a slender brass rod of a length 

 sufiicient to reach within 8 inches of the other extremity of the tube. Into the top 

 of the other brass cap was fastened a brass rod, like the former, only of 8 inches in 

 length. Thus the extremity of one of these brass rods might at pleasure, with- 

 out letting in the air, be made to touch the other ; and for the better observing 

 what difference in effect would arise from an increase of surface, a small brass 

 circular plate was made to screw into each of these extremities. The intent of 

 being able to bring the extremities of these rods near together, and to separate 

 them again to what distance you pleased, was, that it might without difficulty 

 be determined, whether, and to what distance, the electrical fluid would ma- 

 nifest itself in vacuo, farther than in air of the same density with the external. 



The tube then thus fitted, and made dry both within and without, was placed 

 in a cylinder of brass, of about 2 inches long, and of a diameter just sufficient to 

 admit the brass cap before mentioned ; and round the rim of this brass cylinder, 

 to prevent the ingress of air, was adapted a narrow piece of wet leather. These 

 being placed on the plate of the air-pump, which stood upon cakes of wax, a 

 piece of wire passed from the prime conductor to the long brass rod, at the other 

 extremity of the tube, and by these means, on setting the electrical machine in 

 motion, the long brass rod in the tube was electrified. When the brass plate at 

 the bottom of this rod was placed near, or even at the distance of 2 inches from 

 the plate of the other rod, the brushes of electrical fire were seen passing from 

 the periphery of the upper plate to that of the lower, and every part of the air 

 pump snapped on the touch of any one standing on the floor, and gave the other 

 usual signs of the accumulation of electricity. But, as these plates were made 

 to recede from each other, this effect grew less and less ; so that when they were 

 removed 5 or 6 inches from each other, no snaps could be drawn from the air- 

 pump ; as the dissipation of the electric fluid was now as easy from every part of 

 of the prime conductor, as from the upper brass plate in the tube. 



On exhausting this tube, and electrizing as before, the air-pump still standing 

 upon cakes of wax, the electrical fire was not only seen to pass from one plate to 

 the other at the distance of 5 inches, but the same effect ensued at the greatest 

 distance, to which in the tube the brass plates could be drawn. Being therefore 

 desirous to see a further effect, and to avail himself of the whole length of this 

 tube, Mr. W. took from the inside of it the short brass rod, to which the lower 

 brass plate was fixed, and fastened this plate at the very bottom of the tube into 



V. 



