1 3 2 SECTION PHYSICS. 



are in the open sea and there is no place to stop in order to take up 

 the two stations, a thing which it is impossible to do by any other 

 means, or at least has never yet been done, as the sailors tell me. 



" Finally, we shall know with perfect accuracy the distance from us 

 of the clouds from which issues the thunder, and this by measuring with 

 the same pendulum the time which escapes from the moment when 

 the flame of the discharge or the beginning of the lightning is first 

 seen to the instant when the sound reaches the ear ; for, by the Rule 

 of Three, we shall say that if in forty vibrations exactly three miles 

 (that is, 9000 braccia) are traversed by sound, how many miles or 

 braccia will be traversed in ten, twelve, or thirty vibrations ; for the 

 fourth number given will show the distance required. 



" It is well to notice that the measurements of very great distances 

 will prove more accurate than those of the nearer ones, and this be- 

 cause the interposition of time between the flash and the arrival of the 

 sound is so short that there is but little time to measure the vibrations 

 even of the shortest and therefore quickest pendulum, which would be 

 the one it would be necessary to use in similar operations, &c. 



"VlNCENZO VlVIANI. 

 " October 10, 1656." 



" It so happened that their Highnesses thoroughly approved of these 

 ideas and became all the more anxious to endeavour to find by means of 

 experiments the proportion of these velocities. Shortly afterwards Signer 

 Borelli appeared, and his Highness commanded that the next evening we 

 should go to make the trial on the high road of the wood of St. Moro 

 dalle Mulina, and appointed Ricci and Monsu Filippo to aid us. We 

 went on the appointed day at 20 o'clock, having with us the same 

 bombardier, who was the lame De Neri who had fired the shots at the 

 Petraja, carrying with him powder, rockets, and a cannon (maschid). 

 We employed the hours of the day in accurately measuring the length 

 of the road, and we found that from the old wood to the Arno it was a 

 little more than a mile and one-fifth ; but in order that the experiment 

 should be accurate we measured out exactly one and one-fifth mile, 

 and at that spot we made the cart stop with the maschio on it. We 

 then placed two pendulums, one at the very end of the above-mentioned 

 road, the other at the precise middle ; they were instruments the 



