506 The Book of the Balance of Wisdom . [October, 
fully turned off by the lathe. It has two bases, at its two 
ends, resembling two light drum-skins, each fitted to the 
end, carefully, with the most exaCt workmanship ; and on 
the inner plane of one of the two bases is a piece of tin, 
carefully fitted to that plane by the lathe, shaped like a 
funnel, the base of which is the drum-skin itself. The in- 
strument being thus made, when put into liquid in a reser- 
voir or vessel, it stands upon it in an ereCt position and does 
not incline any way.” 
The author then describes at length the manner of gra- 
duating the instrument, the decimal system being employed 
throughout. He remarks that the weight of the funnel- 
shaped piece of tin must be varied according to the density 
of the water assumed as a standard. Tables of the specific 
gravities corresponding to the marks on the instrument ac- 
company the detailed account of its application. 
The annexed figure of the hydrometer of Pappus (Fig. 2) 
does not give a very clear idea of the instrument, and is in- 
tended to show chiefly the manner of construdting and 
graduating it. The details given in the manuscript are so 
minute, however, that it is evident Pappus’s instrument re- 
sembled closely that of Gay-Lussac. It was, however, pro- 
vided with two scales, one with its numbers increasing 
upwards to indicate the volume submerged in liquids of 
different density ; the other with its numbers increasing 
downwards, to show the specific gravities corresponding to 
those submerged volumes. 
The above-mentioned Pappus was an eminent Greek 
geometer of Alexandria, who flourished about 380 or 400 a.d. 
Consequently he was a contemporary of Synesius of Cyrene 
(378—430 A.D.), in one of whose letters occurs what is ordi- 
narily regarded as the first recorded mention of the hydro- 
meter. It is certainly most interesting to find that 
al-Khazini’s description of Pappus’s instrument corresponds 
very closely with the statements of Synesius— a coincidence 
not observed by previous writers. 
Synesius, “ the good bishop of Ptolemais,” writing to his 
instructress,* the fair Pagan philosopher and mathematician, 
the ill-fated Hypatia, and being desirous of trying the winesf 
he is using, says— 
“ My health is so delicate that I need a hydroscope, and I 
beg you to have one made for me of copper. It is a tube, 
cylindrical in shape, and of the form and size of a pipe ; on 
* Not in a letter of Hypatia to Synesius, as Hoefer has it in his Hist. Phy» 
sique ; Paris, 1872, i2mo. 
f Draper, Hist. Int. Devel. Europe, 
