This explanation of the Montlhéry echoes is an inference from observations made at Villejuif. The inference requires qualification. Some hundreds of cannon-shots have been fired at the South Foreland, many of them when the heavens were completely free from clouds, and never in a single case has a _roulement_ similar to that noticed at Montlhéry been absent. It follows, moreover, so hot upon the direct sound as to present hardly a sensible breach of continuity between the sound and the echo. This could not be the case if the clouds were its origin. A reflecting cloud, at the distance of a mile; would leave a silent interval of nearly ten seconds between sound and echo; and had such an interval been observed at Montlhéry, it could hardly have escaped record by the philosophers stationed there; but they have not recorded it. I think both the fact and the inference need reconsideration. For our observations prove to demonstration that air of perfect visual transparency is competent to produce echoes of great intensity and long duration. The subject is worthy of additional illustration. On the 8th of October, as already stated, the siren was established at the South Foreland. I visited the station on that day, and listened to its echoes. They were far more powerful than those of the horn. Like the others, they were perfectly continuous, and faded, as if into distance, gradually away. The direct sound seemed rendered complex and multitudinous by its echoes, which resembled a band of trumpeters, first responding close at hand, and then retreating rapidly toward the coast of France. The siren echoes on that day had 11 seconds’, those of the horn 8 seconds’, duration. In the case of the siren, moreover, the reinforcement of the direct sound by its echo was distinct. About a second after the commencement of the siren-blast the echo struck in as a new sound. This first echo, therefore, must have been flung back by a body of air not more than 600 or 700 feet in thickness. The few detached clouds visible at the time were many miles away, and could clearly have had nothing to do with the effect. On the 10th of October I was again at the Foreland listening to the echoes, with results similar to those just described. On the 15th I had an opportunity of remarking something new concerning them at Dungeness, where a horn similar to, but not so powerful as, those at the South Foreland, has been mounted. It rotates automatically through an arc of 210°, halting at four different points on the arc and emitting a blast of 6 seconds’ duration, these blasts being separated from each other by intervals of silence of 20 seconds.