VOL. LXVIII.] PHILOSOPHICAL TRANSACTIONS. 345 



length, and something less than 1 inch in diameter, was fixed a ball of the same 

 matter. The larger of these measured 3 inches in diameter, and the lesser l-|2g- 

 inch. The larger ball was then brought usually within IJ- inch of another ball, 

 l-jSj-inch in diameter. Being so prepared, the model was set on a table, directly 

 under the ball at the remote end of the less substitute, with the pointed con- 

 ductor on it : and all the wires were properly connected, so as to make a free 

 communication between the model and the well. Nothing now remained but to 

 put the machine in motion ; when, after 10 turns of the wheel, the point on the 

 model was struck at the distance of 4 inches. In the l^th experiment, where 

 the model was in motion, the point was struck at the distance of 5 inches nearly 

 from the cylinder. This difference of distance in these 2 experiments seemed to 

 arise chiefly from a difference in the states of the air, it being rather unfavour- 

 able when the 1 8th experiment was tried. 



Exp. ig. However, some time after, when the state of the air was very 

 favourable, I repeated the last experiment, and observed that the point was 

 struck at 6\ inches. 



Exp. 20. I now repeated the 18th experiment; and attending only to the 2 

 cramps at the corner, there appeared, at the instant when the point was struck, 

 a small spark or explosion between them : which clearly showed, that the stream 

 like explosion, observed in the 17th experiment, was only this small spark ac- 

 companied with the circumstance of the motion of the model. 



\Oih Observation. — From what we have now experienced it appears, that thun- 

 der-clouds, even at rest, and that strike each other at a given distance with the 

 matter of lightning, occasion the same phenomena nearly which a single cloud 

 produces when motion is introduced. 



Exp. 21. When the distance between the two substitutes was made less in 

 any degree than the greatest striking distance, it made a considerable difference 

 in the effects ; because the fluid in these cases passed more freely from the 

 greater to the less substitute : and the freer it passed into the latter, the nearer 

 they approached to be one substitute. So that bringing the 2 substitutes into 

 contact, occasioned the same phenomena that the great cylinder did alone : that 

 is, the rounded end would cause an explosion at a considerable distance ; and the 

 point little or none, though it was brought almost close to the substitute. — Exp. 

 22. But if motion in this case was introduced, during the contact of the 1 sub- 

 stitutes, the point was struck at g^ inches distance from the ball. The motion 

 employed on this occasion was by the hand only, which held a proper stand with 

 the point upon it ; and this point commvmicated by a wire with the well. 



llth Observation. — Now the nearer the 2 substitutes were brought together, 

 the nearer they represented one cloud ; and consequently the niatter of light- 

 ning would pass from one to the other in these cases more readily, and without 



VOL. XIV. Y Y 



