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pitch of an ordinary building supplemented with openings, 
and these openings on different plans, for allowing the steam 
or vapour to escape. The usual one being made thus : — 
This form seems to send out a large amount of steam or 
vapour from the quantity of cold air which is mixed with the 
vapour inside and escapes with it. If one watches the action 
of the cold air and vapour in this form of roof, it w r ill be 
observed, that the cold air comes in by all the windward 
openings, strikes the hot moist air or vapour, drives part out 
and forces a great part down into the building. I found by 
taking off the side lifts, and leaving only the top ones, that 
an improvement was effected. Still, however, the same evils 
were produced by the top louvre boards, although in a less 
degree, owing to a less quantity of cold air being admitted. 
I also tried removing the top cover, hood, or louvre boards, 
and contracting the opening, leaving three feet all along the 
top of the roof without any cover whatever; and this, in 
spite of the admission of rain, I found to be the best of the 
old form. Even with this, however, we had always drops 
and an obscure atmosphere, except in bright dry days. But 
with this form the number of drops were less and the building 
clearer than with so many openings. 
With the old form of roof the most successful plan of keep- 
ing the building free from vapour or drops, is to cover over 
every vessel from which vapour arises, and to have a tube 
passing from each cover quite through the roof. But this 
plan has several objections to it ; the chief being, that owing 
to the boxing in of the winces, &c., one does not see how the 
goods are going on, and whether the processes are being con- 
ducted successfully or not. The workmen also aggravate 
