[65] Sir John Herschel gives the following account of Arago’s observation: “The rolling of thunder has been attributed to echoes among the clouds; and, if it is considered that a cloud is a collection of particles of water, however minute, in a liquid state, and therefore each individually capable of reflecting sound, there is no reason why very large sounds should not be reverberated confusedly (like bright lights) from a cloud. And that such is the case has been ascertained by direct observation on the sound of cannon. Messrs. Arago, Matthieu, and Prony, in their experiments on the velocity of sound, observed that under a perfectly clear sky the explosions of their guns were always single and sharp; whereas, when the sky was overcast, and even when a cloud came in sight over any considerable part of the horizon, they were frequently accompanied by a long-continued roll like thunder.”—“Essay on Sound,” par. 38. The distant clouds would imply a long interval between sound and echo, but nothing of the kind is reported. [66] A friend informs me that he has followed a pack of hounds on a clear calm day without hearing a single yelp from the dogs; while on calm foggy days from the same distance the musical uproar of the pack was loudly audible. [67] The horn here was temporarily suspended, but doubtless would have been well heard. [68] Experiments so important as those of De la Roche ought not to be left without verification. I have made arrangements with a view to this object. [69] The Elder Brethren have already had plans of a new signal-gun laid before them by the constructors of the War Department. [70] Described in Chapter V., p. 229. [71] The figure is but a meagre representation of the fact. The band of light was two inches wide, the depth of the sinuosities varying from three feet to zero. [72] In his admirable experiments on tuning, Scheibler found in the beats a test of differences of temperature of exceeding delicacy. [73] Sir John Herschel and Sir C. Wheatstone, I believe, made this experiment independently. [74] A subject to be dealt with in Chapter IX. [75] Nor indeed any of those tones whose rates of vibration are _even_ multiples of the rate of the fundamental. [76] According to Kolliker, this is the number of fibres in Corti’s organ. [77] The comparison employed by Mr. Sedley Taylor appeals with graphic truth to a mountaineer. Considering, the above curve to represent a mountain-chain, he calls the discords _peaks_, and the concords _passes_. [78] This supposition is of course made for the sake of simplicity, the real period of oscillation of a pendulum 28 feet long being between two and three seconds. [79] This figure corresponds to the interval 15:16. For it and some other figures, I am indebted to that excellent mechanician, M. König, of Paris. [80] For some beautiful figures of this description I am indebted to Prof. Lyman, of Yale College.