.J 14 THE POPULAR SCIENCE MONTHLY. 







were specially directed to the point, a slight prolongation of the can- 

 non-sound might well escajje observation; and it would be all the 

 more likely to do so if the eclioes were so loud and prompt as to form 

 aj^parently part and parcel of the direct sound. 



I should be very loath to transgress here the limits of fair criticism, 

 or to throw doubt, without good reason, on the recorded observations 

 of an eminent man ; still, taking into account what has just been stated, 

 and remembering that the minds of Arago and his colleagues were 

 occupied by a totally diflferent problem (that the echoes were an inci- 

 dent rather tlian an object of observation), I think we may justly con- 

 sider the sound which he called "instantaneous" as one whose aerial 

 echoes did not difterentiate themselves from the direct sound by any 

 noticeable fall of intensity, and which rapidly died into silence. 



Turning now to the observations at Montlhery, we are struck by 

 the extraordinary duration of the echoes heard at that station. At the 

 Soiith Foreland the charge habitually fired was equal to the largest of 

 those employed by the French philosophers ; but on no occasion did 

 the gun-sounds produce echoes approaching to twenty or twenty-five 

 seconds duration. It rarely reached half this amount. Even the 

 siren-echoes, which were far more remarkable, more long-continued 

 than those of the guns, never reached the duration of the Montlhery 

 echoes. The nearest approach to it was on the 17th of October, 1873, 

 when the siren-echoes I'equired fifteen seconds to subside into silence. 



On this same day, moreover (and this is a point of marked sig- 

 nificance), the transmitted sound reached its maximum range, the gun- 

 sounds being heard at the Quenocs buoy, which is 16^ nautical miles 

 from the South Foreland. I have already stated that the duration of 

 the air-echoes indicates " the atmospheric depths " from which they 

 come.' An optical analogy may help us here. Let light -fall upon 

 chalk, the light is wholly scattered by the superficial particles ; let the 

 chalk be powdered and mixed with water, light reaches the observer 

 from a far greater depth of the turbid liquid. The chalk typifies the 

 action of exceedingly dense acoustic clouds ; the chalk and water that 

 of clouds of moderate density. In the one case we have echoes of 

 short, in the other, echoes of long duration. These considerations pre- 

 pare us for the inference that Montlhery, on the occasion referred to, 

 must. have been surrounded by a highly-diacoustic atmosphere; while 

 the shortness of the echoes at Villejuif shows the atmosphere sur- 

 rounding that station to have been acoustically opaque. 



Have we any clew to the cause of the opacity ? I think we have. 

 Villejuif is close to Paris, and over it, with the observed light wind, 

 was slowly wafted the air from the city. Thousands of chimneys to 

 windward of Villejuif were discharging their heated currents, so that 

 an atmosphere non-homogeneous in a high degi'ee must have sur- 

 rounded that station. At no great height in the atmosphere the equi- 



"Philosophical Transactions," 1874, Part I., p. 202. 



