4o6 Gazemder.—Rcd Crayons. 



This fourth refervoir alfo receives the water it contains from the fecond vefTel by a 

 tube for that purpofe. It receives the gas from the third by a bended tube, plunged be- 

 neath the water it contains, through which the gas pafles by a cap, like that of a water- 

 ing pot. 



Graduated femicirctes, into the defcription of which Mr. Halle does not enter, are 

 employed, to fhew the precife ftate of the contained fluids. 



In Plate 17, the numbers 1,2, 3, 4 reprefent the refervoirs. A is the pipe througTi 

 which gas is conveyed into the third refervoir. ; BBB is the bended tube through which 

 the fame gas is conveyed from the third refervoir into th e fourth ; C the perforated cap 

 through which the gas is pafled beneath the water of the fourth refervoir. D, the tube 

 through which the other gas is conveyed into the fourth refervoir, to be mixed with the 

 firft — a, pipe of communication to convey water from the firfl refervoir to the fecond . 

 b, pipe of communication from the external air of the fecond refervoir to the top of the 

 ftrft ; c, bottle communicating with the firft refervoir ; d, pipe of communication from 

 the bottle to the firft jefervoir ; e, bended tube plunged in the water of the fecond re- 

 tervoir ; f, fyphon which delivers water from the bottle into a fmall cap fufpended from 

 the neck of the bottle, which bottle itfelf is fupp<^rted by a conneftion with the firft 

 refervoir ; g, pipe through which water is tranfmitted from the fecond refervoir to the 

 third; h, pipe through which water paflijs from the third refervoir to the fourth; k» 

 cock for difcharging the water of the three refervoirs ; 1, level for the fecond refervoir ; 

 jn, level for the third refervoir ; n, real level for the fourth refervoir ; o, level of pref- 

 fure for the fourth refervoir ; p, level of reaftion for the fame refervoir ; q, graduated 

 femicircles to fhew the ftate of the contained fluids. 



VII. 



InJlruSions for making Red Crayons, By A. F. Lomet.* 



JLT is not without difficulty that Crayons of a good quality can be obtained for 

 drawing-fchools, particularly at a diftance from the metropolis. The native ochre 

 fawed into pieces, which are commonly ufed, is almoft always hard and of an uneven 

 and gravelly confiftence ; fo that the outlines in drawing, for which it is ufed, cannot 

 be made either with the foftnefs or the precifion requifite to produce the defired effeft. 

 The only good crayons that can be (procured have been hitherto manufaftured in Paris 

 exclufively, where they have been long fold at a very high price. The heft are known 

 by the naff.e of crayons de pate du citd. Defmarejl, who was probably the inventor. None 



* Annates dc Cliimie xxx. S93. 



ef 



