Diacoustic

The archbishop of Armagh (in Nᵒ 156, Philos. Trans.) compares hearing with vision; and as the latter is divided into direct, reflected, and refracted vision, so in like manner he divides the former into three parts, and considers not only direct and reflected sounds, which have been long known, but also refracted ones: for, as he observed that former ages had, in a great measure perfected the doctrine of vision by optic, catoptric, and dioptric inventions, so likewise he did not doubt, but that the doctrine of sounds, both with respect to the object, and the medium or organ, might be broutht to perfection by acoustic, catacoustic and diacoustic, or ( as he denominates them both ways ) by phonic, cataphonic, and diaphonic instruments.

The only hypothesis left therefore is that of diacoustic refraction; by which the sound-beam from one origin is bent and lifted over the observer, while from an opposite origin the refraction is in a reversed direction; and such a quality in the moving air is referable to no other observed condition but that of its motion, that is to the influence of the wind.

These considerations prepare us for the inference that Montlhe/ry, on the occasion referred to, must have been surrounded by a highly-diacoustic atmosphere; while the shortness of the echoes at Villejuif shows that the atmosphere surrounding that station must have been, in a high degree, acoustically opaque.