24>i Mr T. Tredgold on Steam-Boats. 
force is greatly increased, and it is rendered almost perfectly 
transparent. In this state, it forms, for various purposes, a 
good and cheap substitute for glass. It is pliant, quite imper- 
vious to water, and no ordinary wind is capable of destroying it. 
In the construction, for example, of such habitations as Fort 
Enterprise, this easily transported substitute for glass might have 
added greatly to the comfort of Captain Franklin, and his party, 
in whose wellfare all felt and again feel so lively an interest. 
The substance in question, when properly varnished, is abun- 
dantly translucent, but it is not perfectly transparent ; and 
hence distant objects cannot be distinctly seen through it. This, 
however, is not always a disadvantage, as when the admission of 
light, to the exclusion of the external air, is the chief or only 
object. 
When embued with a sort of varnish, composed of boiled 
linseed oil, litharge, and oil of turpentine, it still absorbs the 
humidity of the atmosphere ; but mastick, and other varnishes, 
completely exclude water, whether in the state of a liquid or 
vapour. In its original state it far surpasses oiled paper, for 
the purpose of copjung drawings ; and as it admits of using the 
black lead pencil, &c. equally well after as before it has been 
varnished, it is admirably fitted for supplying the place of the 
ground glass of the camera obscura , and for other purposes in 
the arts ; more especially, if, previously to the application of a 
colourless varnish, it has been subjected to the operation of the 
bookbinder’s hammer. When a steel point is used as a pencil 
in copying drawings, the effect produced is that of figures ground 
on glass. 
Duncan Street, 
Drummond Place, 
Edinburgh. 
Art. VI . — On Steam-Boats. By Thomas Tredgold, Esq. 
Civil Engineer, and Honorary Member of the Institution of 
Civil Engineers of London, &c. 
The application of the Steam-engine to propel vessels at sea, 
is one among the many great improvements which characterise 
the age, and invites us to look forward to a facility of intercourse 
