INDIGO. 325

bates mentioned by Vitruvius , is given, as it was consideredthat this article would have been incomplete without it.

“ It is impossible,” says he, “ that the hjdroscopium 5could be the level or chorobates of Vitruvius , for the lines011 the latter were perpendicular to the horizon, whereasthe lines on the former were parallel to it. The hydroscopitimwas undoubtedly an hydrometer of the simplest construction.The tube may be made of copper, and open at the top ;but at the other end, which, when used, is the lowest, itmust terminate with a cone, the base of which is added tothat of the tube. Lengthwise, along the tube, are drawntwo lines, which are intersected by others, and the morenumerous these divisions are, the instrument will be somuch the more correct. When placed in water, it sinks toa certain depth, which will lie marked by the cross lines,and which will be greater in proportion to the lightness ofwater .” 6 A ligure, which is added , 7 might have been dis-pensed with. When a common friend of FermaU andPetau showed it to the latter, he considered it to be so just,that he wished to have an opportunity of introducing it ina new edition.

Mersenne , we understand, disputed the description givenby Fermat, and although known to him, never mentionedbis name; nor indeed olid he allude to his illustrative des-cription, but his objections appear to be without foundation,frivolous and groundless, easy to be confuted by the leastknowledge of nature and of ancient customs. Want ofroom and their unimportance, prevents our submitting themto the public.

INDIGO.

Indigo is a pigment, and, as its name imports, the pro-duce of that portion of the Asiatic continent called India .

Probability greatly favours the belief that it was broughtto Europe as early as the period of Dioscorides and Pliny ,where it was used as a blue pigment for dyeing and painting.Itis most true, all blue colours which were then and which havesince been used, might not have been this Indian production;

5 See Journal dcs S^avans, for Jan. 1679.

6 Fermat here remarks, that poirr, , which has been desired to be changed,should be retained, but that it should not be translated by librumentum but momen-tum . In a mechanical sense it signifies the weight; and on this account the book*of Archimedes de /Equiponderantibiis are called

7 See also Journal des S^avaus.