On the Improvement in forming Electrical Planispheres. 293 
there is reason to believe that the purification of coal gas, the- 
application of which is daily increasing as a substitute for pro- 
curing light, might be effected in a more ceconomical manner, 
by causing the gas to traverse ignited iron vessels, than by the 
application of quick-lime. The subject is worthy of a strict ex- 
amination, both in a philosophical point of view, as well as with 
regard to practical utility. I have the honour to be, sir, 
Your obedient servant, 
Liverpvol, Sept. 3, 1818. S. PARKER. 
XLVI. Improvement in the Method of forming Electrical 
Planispheres. By Mr. Rowan Hit. ~ 
To Mr. Tilloch. 
Sir, — Nearty four years ago I had occasion to represent 
some of the constellations and other figures by electricity. Hi- 
therto this had always been done upon glass; but 1 wished to con- 
struct the constellations on a scale which required larger plates 
of glass thau I conld conveniently procure. Paper being a nofi- 
conductor of electricity, ] was induced to make trial of it, and 
found that it answered my purpose exceedingly well. At that 
time I constructed four constellations, viz. the Great Bear, the 
Great Dog, the Ship, and the Scorpion. I made use of that 
kind of paper which is called Bristol board; the tin-foil may be 
stuck upon it in the same manner as it is fastened to glass. I 
also found gold size (a liquid used by the gilders) to be very well 
adapted to that purpose; and the figures may be secured more 
completely by covering the whole with a coat of varnish. Since 
that time I have represented upon paper a considerable portion 
of the southern sky. 1 took several sheets of drawing-paper, 
pasted the edges together, and stretched the whole upon a circu- 
Jar wooden frame four feet in diameter. Upon this apparatus are 
represented all the stars of the four first magnitudes within forty 
degrees of the south pole. 
In order to give to the stars of the different magnitudes their 
proper degrees of relative brightness, I took the following me- 
thod. 
For the stars of the first magnitude, I cut the ends of the tin- 
foil round, and placed them about one- twelfth of an inch asunder, 
For those of the second magnitude, the bits of tin-foil were 
pointed, and the spaces between them made as small as possible. 
To produce a spark of no greater brightness than the stars of 
the third magnitude, I made the spaces in the tin-foil similar to 
the last, and pasted over each a small bit of thin paper, through 
T3 which 
