1822.] Cambridge Philosophical Transactions, Part LI. 63 
its density is, therefore, about one-sixth, that of the atmosphere 
being unity. 
According to Mr. Cecil, if the external air be prevented by 
a proper apparatus from returning into this imperfect vacuum, 
the pressure of the atmosphere may be employed as a moving 
force, nearly in the same manner as in the common steam engine ; 
the difference consisting chiefly in the manner of forming the 
vacuum. 
ir, Cecil then enters into an estimate of the power resulting 
from such a vacuum by comparing the effects of equal bulks of 
sicam and hydrogen ; this it will be impossible to comprehend 
without the diagram by which it is illustrated ; but the author 
concludes, that “ it appears by calculation that any quantity of 
pure hydrogen gas will produce more than five times the effect 
of the same bulk of steam; and in practice the disproportion of 
their effects is still greater. It is here supposed, that steam 
produces by condensation a perfect vacuum equal to its own 
bulk ; but this is far from being the case : much of the power is 
lost by needless condensation by the escape of steam through 
the piston, besides a considerable deduction for working an air 
pump, and two water pumps, which are necessary to a steam 
engine. 
This paper is accompanied with a drawing and explanation of 
a model of a gas engine. The drawings are adapted to the 
{sometrical Perspective of Prof. Farish. There is also a draw- 
ing of one of a different construction which Mr. Cecil has intro- 
duced on account of its simplicity. 
The paper concludes with some observations upon the use of 
the explosive force of gunpowder as a moving force, and with 
showing that it cannot be practically useful, for several reasons, 
and particularly from the corrosion of metals by the sulphur 
contained in the gunpowder, and by the sulphuric acid which is 
produced during combustion. 
IV. Ona remarkable Peculiarity in the Law of the extraordi= 
nary Refraction of differently-coloured Rays exhibited by certain 
Varieties vs Apophyllite. By J. F. W. Herschel, Esq. FRS. of 
London, Edinburgh, and Gottingen, &c. &c. 
In this paper Mr. Herschel refers to the figures contained in 
the first part of the Transactions ; and as without these, it would 
be imperfectly intelligible, we shall not attempt any analysis of 
this paper. A 
(To be continued.) 
