But, returning to our former point of view, and looking upon these centuries of terrible blood-shedding as the fierce, furious war which the Roman Empire waged against God and his religion, we naturally ask ourselves a question, Where is the great God of the Christians whilst his children are being immolated to pagan savagery throughout the whole earth? Does he from his high heaven take note of what is done? Oh! he who sees the sparrow fall does not lose sight of his children, nor does his eye fail to see the sufferings which they endure for him. The voice of his martyrs rose heavenwards with a mighty cry during those three hundred years. It rose from the saturated floor of the Roman amphitheatre; it spoke with pleading eloquence from the depths of the mines of Numidia; it echoed incessantly in the ear of God from amid the solitudes of Pannonia. God was not at any time deaf to that cry. He was slow in his anger, but, then, on that account he was the more terrible. Whilst Nero was shedding the first Christian blood at Rome, God was silently gathering together his avenging armies in the forests of the north. It took him more than three hundred years to marshal his overwhelming warrior-hosts; but, O heavens! what a direful shaking of the universe when they did come!
ACOUSTICS AND VENTILATION. [52]
Every effort to elucidate what is obscure, or to provide a remedy for acknowledged evils, is a just title to that friendly acknowledgment which the writer of this little book bespeaks. It is a step in the direction of progress. But it is of the highest importance in the attempt to impart clear ideas upon any subject, that they should be so distinctly expressed as to leave no doubt concerning the identity of their subject. Thus, in treating of sound, it seems to us that the question first presented is this: What is sound? Our author says that it “receives its vitality or its life through the air, and without air sound loses it and becomes extinct.”
We object to this statement of the origin of sound, as both unsatisfactory and indistinct. It implies that sound is something born and floating in the air, and external to the mind perceiving. We fancy that, without an ear to hear, sound would not become extinct, but have no existence; and that the vitality of which our writer treats is not in or on the air, but in the mind itself. This exception to the supposed origin of the life of sound may not seem to affect the discussion of acoustics as far as the practical purpose of the architect is concerned; but we insist that neither the drumsticks nor the drum, nor the air within it or without, nor even all these at work, are sound, more than the telegraph wire and the electric current are the message sent from one operator to another.
That inaccuracy which we discover in our author’s use of terms, we find also in his quotations from others. For example: “The intensity of sound depends on the density of the air in which the sound is generated, and not on that of the air in which it is heard. A feeble sound becomes instantly louder as soon as the air becomes more dense. So you will always find, on great elevations in the atmosphere, the sound sensibly diminished in loudness. If two cannon are equally charged, and one fired at [from] the top of a high mountain, and the other in a valley, the one fired below, in the heavy air, may be heard above, while the one fired in the higher air will not be heard below; owing to its origin, the sound generated in the denser air is louder than that generated in the rarer. Peals of thunder are unable to penetrate the air to a distance commensurate with their intensity on account of the non-homogeneous character of the atmosphere which accompanies them; from the same cause, battles have raged and have been lost within a short distance of the reserves of the defeated army, while they were waiting for the sound of artillery to call them to the scene of action.”
It seems to us that the truth here expressed is not unmixed with error. In the very first sentence, we think that accuracy would require the suppression of the word not. The intensity of sound depends not only upon the density and elasticity of the air whose pulsation is an antecedent condition, but also upon the density and elasticity of the air through which the pulse is transmitted. While it is true that a pulse given to the denser column or stratum of air may be transmitted through a rarer medium with greater resultant force than if its origin and direction were reversed, it by no means follows that the intensity of sound is unaffected by the density of the air in which it is heard. We apprehend the truth to be that the pulse given to highly rarefied air is very feeble; and its secondary effect upon a denser and more elastic fluid, correspondingly slight; while the pulse from the denser air would be transmitted with greater—but still diminished—force, through the rarer atmosphere in which it reaches the ear. An absolute vacuum could not transmit the pulse given through a column or stratum of elastic fluid. A rarefied atmosphere could but transmit it with a force always varying with its own elasticity. And were it possible to preserve one’s consciousness within the exhausted receiver of an air-pump, we doubt if the most sensitive ear could be made to hear the roar of a cataract without.
“A feeble sound becomes instantly louder as soon as the air becomes more dense;” but not as loud as if the same initial pulse were immediately given to the denser air. In the case of two cannon equally charged, one of which is fired on the top of a mountain, and the other in a valley below it, to say that “owing to its origin, the sound generated in the denser air is louder than that generated in the rarer,” sounds much like saying it is because it is. If it be more than this, it is wrong. It is a clear case of non causa pro causa. The origin [of the pulse] of sound is in either case the same: the explosion of equal charges of gunpowder, in guns supposed to be of like material and equal size. The effects are not the same, because the effect of a force depends upon its transmission as well as upon its origin.
Does the atmosphere “accompany” peals of thunder? Or does this expression convey a distinct idea of the office of the atmosphere in the production of sound? We understand that the atmosphere receives the pulse or blow, and that its transmission to the ear is due to the elastic force of the intermediate air. It is not the homogeneousness of air, but its elasticity which transmits the pulse. And though, in architecture, the object sought is a uniformly elastic air throughout the auditorium, it does not follow, nor is it even desirable, that the maximum effect at a given point should be obtained by it.