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Philo was a civil engineer, whose dates and works are disputed. He may have flourished mid 2nd century BC, or mid 3rd century BC; and he may (or may not) have compiled the first known list (with descriptions) of the Seven Wonders of the World. If he did not, then the description so confidently cited everywhere on the web (two pasted below), actually come from the 6th century AD—unless they were Later Roman.
Whoever wrote it, the description comes from a text called Peri ton hepta theamaton (Concerning the Seven Wonders of the World).
Shortened version from University of South Florida, engineering dept:
The Hanging Garden has plants cultivated above ground level, and the roots of the trees are embedded in an upper terrace rather than in the earth. The whole mass is supported on stone columns... Streams of water emerging from elevated sources flow down sloping channels... These waters irrigate the whole garden saturating the roots of plants and keeping the whole area moist. Hence the grass is permanently green and the leaves of trees grow firmly attached to supple branches... This is a work of art of royal luxury and its most striking feature is that the labor of cultivation is suspended above the heads of the spectators.
Here is a fuller version from English Reading Online:
The Hanging Garden is so called because it has plants cultivated at a height above ground level, and the roots of the trees are embedded in an upper terrace rather than in the earth. This is the technique of its construction. The whole mass is supported on stone columns, so that the entire underlying space is occupied by carved column bases. The columns carry beams set at at very narrow intervals. The beams are palm trunks, for this type of wood—unlike al others—does not rot and, when damp and subjected to heavy pressure, it curves upwards. Moreover it does itself give nourishment to the root branches and fibres, since it admits extraneous matter into its folds and crevices. This structure supports an extensive and deep mass of earth, in which are planted broad-leaved trees of the sort that are commonly found in gardens, a wide variety of flowers of all species and, in brief, everything that is most agreeable to the eye and conductive to the enjoyment of pleasure. The whole area is ploughed in the same way as solid ground, and is just as suitable as other soil for grafting and propagation. Thus it happens that a ploughed field lies above the heads of those who walk between the columns below. Yet while the upper surface of the earth is trampled underfoot, the lower and denser soil closer to the supporting framework remains undisturbed and virgin. Streams of water emerging from elevated sources flow partly in a straight line down sloping channels, and are partly forced upwards though bends and spirals to gush out higher up, being impelled through the twists of those devices be mechanical forces. So, brought together in frequent and plentiful outlets at a high level, these waters irrigate the whole garden, saturating the deep roots of the plants and keeping the whole area of cultivation continually moist. Hence the grass is permanently green, and the leaves of trees grow firmly attached to supple branches, and increasing in size and succulence with the constant humidity. For the root system is kept saturated and sucks up the all-pervading supply of water, wandering in interlaced channels beneath the ground, and securely maintaining the well-established and excellent quality of the trees. This is a work of art of royal luxury [lit: riotous living], and its most striking feature is that the labour of cultivation is suspended above the heads of the spectators. [Finkel, in Clayton & Price, attributes this translation to Professor David Oates.]
For amusement, here is Philo's comment on the Seven Wonders:
Everyone has heard of each of the Seven Wonders of the World, but few have seen all of them for themselves. To do so one has to go abroad ... Only if you travel the world and get worn out by the effort of the journey will the desire to see all the Wonders of the World be satisfied, and by the time you have done that you will be old and practically dead.