606 PHILOSOPHICAL TRANSACTIONS. [aNNO \76g. 



to pass above -J- of an inch in the air, rather than make the circuit of the 

 continued wire. Using a longer and smaller iron wire, the passage through the 

 air exceeded half an inch. He then took 4 or a yards of iron wire -^Vth of 

 an inch thick, when the passage through the air was still half an inch ; and 

 taking 3-^ yards of wire that was -Lth of an inch thick, the spark in the air was 

 half an inch, and sometimes near three quarters of an inch. Making use of only 

 half the length of this wire, the passage through the air was only half that 

 distance, or -j-th of an inch. When he kept the place of near contact about the 

 middle of this wire, and made the explosion at the extremities of the whole 

 wire, he was obliged to bring them about as near again, i. e. to little more than 

 the 8th of an inch, before the passage would be through the air ; so that the 

 force of the whole explosion must have been greatly weakened by its passing 

 through so much of the wire. Lastly, he took a pair of kitchen tongs, the 

 legs of which were 2 feet, and the smallest part of them above half an inch 

 in diameter ; when the circuit was made about -f th of an inch in the air (for at 

 that distance from one another the ends of the tongs had been fixed) rather than 

 through 4 feet of that thick iron. 



Notwithstanding this evident passage of the electric matter through the air, 

 at the same time that a metallic circuit was provided for it ; it was certain that 

 the whole of the charge did not pass in the air : for when he extended -j- of an 

 inch of small iron wire between the nearest parts, it was only made ret-hot by 

 the discharge ; whereas above 2 inches of it would have been exploded, if there 

 had been no other metallic circuit at all. 



As the electric fire meets with so much obstruction in passing through a 

 circuit of iron of this thickness. Dr. P. makes no doubt but that it is consi- 

 derably obstructed in passing through metallic circuits of any thickness whatever; 

 and that it would prefer a very short passage through the air, if they were made 

 even of no great length. In this method the different degrees of conducting 

 power in different metals may be tried, using metallic circuits of the same length 

 and thickness, and observing the difference of the passage through the air in 

 each. N. B. A common jar answers as well, in these experiments, as a large bat- 

 tery. It is evident, from many experiments, that the whole fire of an explosion 

 does not pass in the shortest and best circuit ; but that, if inferior circuits be 

 open, part will pass in them at the same time. Of this Dr. P. made the follow- 

 ing satisfactory trial. He took an iron chain, and laid it upon a table, in con- 

 tact with a charged jar ; so that the parts of it made two circuits for the dis- 

 charge, which he could vary at pleasure ; and he observed that, when one of the 

 circuits was but half an inch, and the other more than half a yard ; yet, if the 

 charge was high, it always went in them both, there being considerable flashes 

 between the links of the remotest part of the chain. If the charge was weak, 

 it passed in the shortest circuit only. 



