S68 Mr Tredgold on the Theoretical Principles 
upon a white colour covering the black, the lines being impressed 
so as to penetrate to the black ground 
The eighth class we shall appropriate to those very rare vases, 
commonly but falsely called Egyptian, in which the ground is 
yellowish, and the paintings of a coifee-colour, which, however, 
does not cover the ground perfectly, there being sometimes a 
covering of white and red colours. These vases, found in Lower 
Italy, correspond, in so far as regards the colour of the clay and 
paintings, with others discovered in Greece, one of which, that 
had been dug up at Athens, is preserved in the Museum of our 
University, having been presented to it by the celebrated Eng- 
lish traveller Haxvhins. 
{To he continued.) 
Art. XIX. — On the Theoretical Principles and Power (yf 
Brown's Gas Machine. By Thomas Tredgold, Civil En- 
gineer, &c. 
^Lhen a machine is proposed to the public as likely to rival 
the steam-engine in power and economy, it becomes necessary 
to investigate its principles, in order to be able to give an answer 
to the inquiries of one’s employers ; and as the subject is of pub- 
lic interest, perhaps a view of the method I followed in the in- 
vestigation of the power and expence to produce a given effect 
with Mr Brown’s gas machine? may not be wholly uninteresting 
to your readers. 
The power of this machine is evidently owing to the air in 
the cylinder being rarified by the combustion of gas within it. 
A similar effect is produced by burning some very inflammable 
paper under an inverted bell glass, and letting it down, when 
full of flame, into a vessel of water ; the water immediately rises 
within the glass, and condenses the rarified air it contained to 
about one-third of its volume. 
Now, let us suppose the temperature of the water used for 
condensation to be 50 degrees, and let t be the temperature of the 
* Jorioy loc. cit. p. 5. 
