terrace.  The stone slab had a channel cut round the margin, to be filled with water in order to determine whether the slab was level or not, and the style was set vertical as in hour-dials.[9] We may suppose this gnomon to have been erected that by its aid the shadow at the solstices and equinoxes might be precisely noted, for in that view both the slab and the style were graduated.  The fourth and last instrument, and the largest of all, was one consisting as it were of three or four huge astrolabes in juxtaposition [D]; each of them having a diameter of such a geometrical pace as I have specified.  The fiducial line, or Alhidada, as it is called, was not lacking, nor yet the Dioptra.[10] Of these astrolabes, one having a tilted position in the direction of the south, represented the equator; a second, which stood crosswise on the first, in a north and south plane, the Father took for a meridian; but it could be turned round on its axis; a third stood in the meridian plane with its axis perpendicular, and seemed to stand for a vertical circle; but this also could be turned round so as to show any vertical whatever.  Moreover all these were graduated, and the degrees marked by prominent studs of iron, so that in the night the graduation could be read by the touch without a light.  All this compound astrolabe instrument was erected on a level marble platform with channels round it for levelling.  On each of these instruments explanations of everything were given in Chinese characters; and there were also engraved the 24 zodiacal constellations which answer to our 12 signs, 2 to each.[11] There was, however, one error common to all the instruments, viz. that, in all, the elevation of the Pole was assumed to be 36 deg..  Now there can be no question about the fact that the city of Nanking lies in lat. 32-1/4 deg.; whence it would seem probable that these instruments were made for another locality, and had been erected at Nanking, without reference to its position, by some one ill versed in mathematical science.[12]

[Illustration:  Observatory Terrace]

[Illustration:  Observatory Instruments of the Jesuits.]

“Some years afterwards Father Matteo saw similar instruments at Peking, or rather the same instruments, so exactly alike were they, insomuch that they had unquestionably been made by the same artist.  And indeed it is known that they were cast at the period when the Tartars were dominant in China; and we may without rashness conjecture that they were the work of some foreigner acquainted with our studies.  But it is time to have done with these instruments.”—­(Lib. IV. cap. 5.)