

ACRE ACROCORINTHUS. 



25 



this, probably, that cork and all soft cellular bodies 

 are bad conductors of sound, as in these the sound 

 must, in passing through the walls of the cells and 

 the air contained in them, change successively from 

 one medium to another. All sounds, whatever be 

 their loudness, or pitch, are transmitted with die same 

 velocity ; a fact most completely proved by every 

 musical performance. Were it otherwise, indeed, 

 this beautiful art could not exist. To make this ap- 

 parent, it is only necessary to consider, that harmony 

 is a combination of different sounds arranged with 

 certain relations of time and pitch. Now, if one 

 sound were transmitted with greater velocity than 

 another, these relations would differ at different dis- 

 tances, or be confounded, except at a single given 

 point. Nay, further ; melody, which is a succession 

 -of single sounds, would not reach different ears with 

 the same relations of time, if the different notes wece 

 not transmitted with equal velocities. Some obser- 

 vations on sound, in very high latitudes, seem to con- 

 tradict the above law of transmission. The seeming 

 anomaly, however, is sufficiently reconciled by sup- 

 posing the different strata of air, through which the 

 sounds, in those instances, were transmitted, in very 

 different hygrometrical or thermometrical states ; 

 which would make corresponding differences in their 

 modulus of elasticity. When a wave of sound meets 

 an elastic surface, it is partly transmitted and partly 

 reflected. This reflection, when it returns back per- 

 pendicularly, is called an echo. That an echo may 

 be distinctly heard, it is necessary that the reflecting 

 surface be at such a distance that the original sound 

 sliall have ceased before the reflected one returns to 

 the ear ; otherwise they will be blended, and the 

 echo not perceived. Hitherto we have considered 

 the propagation of sounds in an unconfined medium, 

 particularly the air, in which the wave of sound can 

 diffuse itself in every direction. When this diffusion 

 is prevented by enclosing the medium in a surface 

 capable of reflecting the wave so that the sound shall 

 be confined to one direction, the transmission from 

 one point to another is much more perfect. Expe- 

 riments have been made in this way, in which a hol- 

 low cylinder, about half a mile long was formed by 

 cast-iron pipes. The sound was transmitted by the 

 air, in this cylinder, with wonderful distinctness. 

 The least whisper, at one end of the cylinder, 

 was distinctly heard at the other end. So perfect, 

 indeed, was the transmission, " that, not to hear, it 

 was absolutely necessary not to speak." Captain 

 Parry and lieutenant Foster made several experiments, 

 luring the northern expeditions, to ascertain the 

 velocity of sound. A table of them is given in a num- 

 ber of the Edinburgh Philosophical Journal. These 

 experiments were made at Port Bowen, by means 

 of a brass six-pounder, over a range of 12,892-89 

 feet. The results given are the mean of four shots 

 in one case, of five in another, and, in the rest, 

 of six shots by each observer. The mean results 

 varied from 12",7617 to 11",7387 and 11",5311 for 

 the time in which the range of 12,892-89 feet was 

 traversed by the sound. At the period of the expe- 

 riment which gave the first of these results, there was 

 a calm ; during the second, the wind was light ; 

 during the third, a strong wind was blowing. The 

 velocity per second, in feet, was, in the first instance, 

 1010-28; in the second, 1098-32 ; in the third, 1118-10. 

 Omitting the last of the ten results (the last above 

 given), on account of the strong wind, the mean 

 of the other nine gives a velocity of 1035-19 feet, 

 at the temperature of 17'72, Fahrenheit. The 

 mean of a table of velocities formed from observa- 

 tions made at Fort Franklin, by lieutenant Kendall, 

 who accompanied captain Franklin, in his second 

 journey to the shore of ' the Polar sea, gives a 



velocity of 1009-28 feet per second, at the tempera- 

 ture of 9-14, Falirenheit. The science of acoustics, 

 like the other physical sciences, has been in a constant 

 state of advancement since the revival of learning. It 

 appears that Pythagoras knew the relation between 

 the length of strings and the musical sounds which 

 they produce. Aristotle was not only aware of this 

 relation, but, likewise, that the same relation subsists 

 between the length of pipes and their notes, and that 

 sound was transmitted by the atmosphere. This 

 constituted the sum of ancient learning in this bra nch 

 of science. These facts were taught by Gallileo, 

 and, moreover, that the difference in the acuteness 

 of sounds depends on the different frequency of vi- 

 brations, and that the same string, if of uniform 

 thickness and density, must perform its vibrations in 

 equal times. But, without attempting a history of 

 modern discoveries in acoustics, we can only men- 

 tion, that the names of Taylor, Moreland, Newton, 

 Daniel Bernouilli, D'Alembert, Euler, Robison, La- 

 grange, Laplace, Chladni, T. Young, and Biot, are 

 all connected with it. Of these, Newton gave the 

 law of transmission, which we have stated in this 

 article, and the correction for heat was made by 

 Laplace. 



ACRE ; a measure of land, containing four square 

 roods, or 160 square poles or perches. The statute 

 length of a pole or perch is 5 yards, or 165 feet ; 

 but the length of a pole, and, therefore, the size of 

 the acre, varies hi different counties in England. 

 The Scottish acre contains also four square roods ; 

 one square rood is 40 square falls. The English 

 statute acre is about three roods and six falls, stand- 

 ard measure of Scotland; or the English acre is to 

 the Scottish as 78,694 to 100,000. The French acre, 

 arpent, is equal to 54,450 square English feet, of 

 which the English contains only 43,560. The 

 Welsh acre contains commonly two English ones, 

 The Irish A. exceeds the English by two roods, 

 10, Vi perches. The U. S. of America use the English 

 statute acre. 



ACRE (Akka, St Jean d'Acre); in the middle 

 ages, Ptolemais, a city and harbour on the coast of 

 Syria, capital of a Turkish pachalic, between the 

 pachalics of Damascus and Tripoli, which contains 

 420,000 inhabitants, and 6275 sq. miles. This city, 

 situated at the foot of mount Carmel, is the chief 

 emporium of Syrian cotton, and contains about 16,000 

 inhabitants ; its harbour, though full of sand-banks, 

 is still one of the best on this coast. At the time 

 of the crusades, A. was the principal landing place 

 of the crusaders, and the seat of the order of the 

 knights of St John as late as 1291 ; hence the French 

 name, St Jean d' Acre. The Turks, unfler Djezzar, 

 pacha of this place, who is iamous for his cruelty, 

 sustained, with the assistance of the British commo- 

 dore, Sidney Smith, a siege of 61 days, by the French 

 army under Bonaparte. After a great loss of men 

 on all sides, the French abandoned the siege. (See 

 Egypt, landing of the French in.) 



ACRIDOPHAGI (GV., from a.xt, a locust, and qdyu, 

 to eat) ; an ancient Ethiopian people, who are said to 

 have fed upon locusts. 



ACROCERAUNIUM ; in snc. geogr. a promontory of 

 Epirus, on which are situated the Acroceraunia or 

 monies Ceraunii. They run between the Ionian sea 

 and the Adriatic, where Illyria ends and Epirus be- 

 gins, and are the modern Monti della Chimera. 



ACROCORINTHUS ; a steep rock, about 2 '00 fret 

 high, near the city of Corinth, of a grey colr-tir, and 

 picturesque form, crowned with the remains of old 

 Venetian fortifications, repaired a little by the Greeks, 

 since the commencement of their revolution. It was 

 famous, in ancient times, for its citadel, and on its 

 top stood, according to Pausanias, a temple of Venus. 



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