of those horizontal fissures created by the moisture eagerly
attracted from a marine atmosphere—an apparent stratification,
which is merely superficial, and only wrought by a long contact
with the air. The opposite drawing (Plate 2), which was carefully
made on the spot, will convey a better idea of the appearance
of these rocks, which evidently supplied the stone for the
aqueduct that strides over them b.
During this short stage.of the ascent, the anthemis hispanica
presented itself, but was confined to a very small space; the
inula odorata extended a little further; the convolvulm arvensis
confined itself to the bottom, and although I did not discover
a trace of the genus during the remainder of the ascent, yet on
arriving at the highest point, in the most exposed situation possible,
I found the convolvulus tricolor. The scabiosa succisa was thinly,
scattered at the foot of the first hill ; the sisymbrium palustre also
confined itself to the bottom of the hill;1 the mentha arvensis grew
at the foot, and frequently presented itself until half way up
the ascent, where it totally disappeared. About midway; there
were some dwarfish tufts of the ulex europoeus; and a great
profusion of the genista viscosa, the eu/phorbia detidroides, and
the atraetylis humilis—the former plentifully, the latter thinly
scattered, were found, with the carduus eriophorus and the
b L’Eveque writes, that the great'arph of the aqueduct is 100 feet, three inches,
wide between the'pillars, and that, froin' the keystone of the arch to the bottom of the
rivulet, its height is nearly 206 feet, and 214 to the parapet. The plan published bv
Wells in 1792, (from that presented to the Marquis of Pombal,) makes the height of
the grand arch 226§ English feet, and the width 108f, I made the height, from the
bottom of the rivulet to the parapet, 222. English feet, by u barometer of Fortin’s,
which marked on the parapet wall 750.66, (thermometer 2ÜJ, thermometer detached,
20], centig.) and on the side terrace under the'great , arch, (5 feet above the bed
of the rivulet,) 756.50, (thermometer 20f, thermometer detached 20§.) The length
of the work at the valley of Alcantara is 2873j feet; the whole length 'of the
aqueduct (from its source at Cqnessas to Lisbon), is 56.3.80 feet, following sixty-five
windings and dingles.