Defcription of an Improved Air- Furnace. 19% 
deed, the rules laid down by thofe who have written on this 
fubjeét, differ fo widely from each other, that we muft fup- 
pofe a great number of circumftances, hitherto overlooked, 
enter into the conftru&tion of a good furnace. Having ob- 
ferved effects produced in that of Mr. Edward Howard, of 
Doughty-fireet, which we have feldom, if ever, feen equalled 
in any other furnace, we thought we could not better oblige 
our chemical readers than by giving a defcription of it. The 
difference in the prefent eaeaaGiied of Wedgewood’s pyro- 
meter pieces, from that ufed by him when he firft invented 
his inftrument, makes it impracticable (unlefs the {eale was 
alfo altered) to meafure correétly the degree of heat obtained ; 
it muft however be at leaft = 160°, and 1s fufficient to run 
down the beft Heffian crucibles. The moft ftriking devia- 
tion from the common confiruétion is in the lower part of . 
the chimney, or flue, being fmaller than the upper, and 
the greater depth of the afh-pit. Mr. Howard thinks that 
fomething depends on the direction of the horizontal fun- 
nel, which in this cafe opens to the north. Ii any of our 
readers fhould confiruét a furnace on this plan, we fhould be 
glad to be informed of the coincidence of effect *. 
Fig. 1. (Plate [V.) A, the cavity or body of the furnace, 
9 inches {quare and 1 foot 7 inches deep to the bars. B, the 
_ath-pit, 1 foot 2 inches, by g inches broad, and 2 feet 8 inches 
deep below the bars: this afh-pit opens to the external air by 
an horizontal funnel C, 6 feet long,  pafling under the floor 
of the laboratory: this not only furnithes a fupply of denfer 
air, but prevents the unpleafant effects of the cold draught 
on the legs and feet of the operator, which happens when the, 
ath- -pit opens into the laboratory. The external opening of 
this funnel is about 2 feet {quare, w vhich gradually contracts 
to the afh-pit in the manner fhewn in the plate. D, the 
aperture of the horizontal flue, 7 inches wide by 2! deep, 
contracting in width to 43 inches, where it enters the ver- 
tical flue or chimney, which is 44 inches. {quare, of which 
width it continues to the height of 7 feet, and is then en- 
larged to 5 inches fquare; which dimenfions it shine cia to 
* Dr. Pearfon's furnace, which is a very powerful one, agrees in fome 
parts of its conftruétion with Mr. Howard's 
the 
