TOL. XLIV.] PHILOSOPHICAL TRANSACTIONS. 347 



All these stones are more or less transparent; the following are all opaque: 

 The cat's-eye, brown. — The red jasper, called also thick cornelian, is of the 

 colour of red ochre. — The jet, black. — Agates, are of various sorts. — The blood- 

 stone, is green, veined or spotted with red and white. — ^The onyx, consists of 

 different parallel strata, mostly white and black. — The sardonyx, of several shades 

 of brown and white. — The agat onyx, of two or more strata of white, either 

 opaque or transparent. — Alabaster, different strata of white and yellow, like the 

 agate-onyx, but all opaque. — The toad's-eye, black. — ^The turquoise, of a yel- 

 lowish blue inclining to green. — Lapis lazuli, is of a fine deep blue. 



Of most of the species beforementioned there are some of an inferior class 

 and beauty. These are commonly called by jewellers occidental stones ; they are 

 mostly the produce of Europe, and found in mines or stone quarries ; and are so 

 named, in opposition to those of a higher class, which are always accounted 

 oriental, and supposed to be only produced in the more eastern parts of our 

 continent. 



The onyx, sardonyx, agate-onyx, alabaster of two colours or strata, as also 

 certain shells of different coats, were frequently engraved by the ancients in relief; 

 and these sorts of engravings are commonly called cameos. They also some- 

 times ingrafted a head, or some other figure in relief of gold, on a blood-stone. 

 Besides which, there are some antiques, mostly cornelians, that are covered with 

 a stratum of white. This stratum has by some been considered as natural, but 

 it was really a sort of coat of enamel that was laid on. This was used only in 

 the times of the lower empire. 



The stones esteemed the best for engravmg on, were the onyx and sardonyx; 

 and next to them the beryl and the jacinth. The ancients engraved most of 

 their stones, except the onyx and the sardonyx, just as they were found: their 

 natural polish excelling all that can be done by art ; but the beauty of the several 

 species of onyxes could only be discovered by cutting. 



The merit both of intaglios and cameos depends on their condition, on the 

 goodness of the workmanship, and on the beauty of their polish. The antique 

 gems of Greek work are the most esteemed, and next to them the Roman ones, 

 in the times of the higher empire. 



A New Method for a Mural Astronomical Quadrant, free from many inconve- 

 niencies to which former ones are subject. By Christian Lewis Gersten, F.R.S. 

 N° 483, p. 507. From the Latin. 



The great usefulness of arches, firmly fixed to walls in the plane of the meri- 

 dian, is well known to all who are the least conversant with astronomical obser- 

 vations. Hence it is that few observatories can be accounted well furnished 

 without one. Yet it is commonly found that there is no wall so solid and firm, 



Y Y 2 



