VOL. XLV.] PHILOSOPHICAL TKANSACTIONS, 44^ 



2 miles, was what connected the short iron rod beforementioned, and the coated 

 phial, with their respective conducting wires. 



The obsei-vers being placed at their respective stations, the observer at the ma- 

 chine proceeded in making the explosions of the coated phial ; by which the ob- 

 servers at the 9-mile stone were very strongly shocked ; and they were also felt at 

 the 7-niile stone. This demonstrated that the circuit here formed by the elec- 

 tricity was 4 miles, viz. 2 miles of wire, and 2 miles of ground, the space be- 

 tween the extremities of that wire. A distance without trial too great to be cre- 

 dited ! how much farther the electrical commotion will be perceptible, future ob- 

 servations can only determine. 



The electrical commotion by the observers near the 7-niile stone was but 

 slightly felt; nor could it be otherwise expected, the wire in many parts of its 

 length touching, as was beforementioned, the moist vegetables; which, in as 

 many places as they were touched, formed subordinate circuits. We find, in all 

 other instances, that the whole quantity of electricity, accumulated in the coated 

 phial, is felt equally through the whole circuit, when every part is in a great 

 degree non-electric; so here the whole quantity, or nearly so, determined that 

 way, was felt by the observers at the Q-mile stone; while those at the other sta- 

 tion felt so much of their quantity only, as did not go through the vegetables; 

 that is, that proportion only in which iron is a greater non-electric than the 

 vegetables. 



Though the electrical commotions, felt by the observers near the 7-niile stone, 

 were not strong; they were equally conclusive in showing the diiference between 

 the respective velocities of electricity and sound. The space through which 

 sound is propagated in a given time, has been very differently estimated by the 

 authors who have written on this subject. Roberval gives it at the rate of 56o 

 feet in a second; Gassendus at 1473; Mersenne at 1474; Du Hamel, in the 

 History of the Academy of Sciences at Paris, at 11 72; the Academy del Cimento 

 at 1185; Boyle at 1200; Roberts at 1300? Walker at 1338; Sir Isaac Newton 

 at 968 ; Dr. Derham, in whose measure Mr. Flamsteed and Dr. Halley acquiesced, 

 at 1142. But by the accounts since published by M. Cassini de Thury in the 

 Memoirs of the Royal Academy of Sciences at Paris for the year 1738, where 

 cannon were fired at various as well as great distances, under great variety of 

 weather, wind, and other circumstances, and where the measures of the difl^erent 

 places had been settled with the utmost exactness, sound was propagated at a 

 medium at the rate only of 1038 French feet in a second. The French foot 

 exceeds the English by 7-l lines, or is as I07 to 114; and consequently 1038 

 French feet are equal to 1106 English feet. The difference therefore of the 

 measures of Dr. Derham and M. Cassini is 34 French or 36 English feet in a 



VOL. IX. 3 M 



