I could gather, besides the mechanical
contrivances of his vessel, he had a chymical liquor, which he
accounted the chief secret of his submarine navigation. For when,
from time to time, he conceived that the finer and purer part of
the air was consumed, or over-clogged by the respiration and
steam of those that went in his ship, he would by unstopping a
vessel full of this liquor, speedily restore to the troubled air
such a proportion of vital parts, as would make it again, for a
good while, fit for respiration whether by dissipating, or
precipitating the grosser exhalations, or by some other
intelligible way, I must not now stay to examine, contenting
myself to add, that having had the opportunity to do some service
to those of his relations that were most intimate with him, and
having made it my business to learn what this strange liquor
might be, they constantly affirmed that Drebel would never
disclose the liquor unto any, nor so much as tell the nature
whereof he had made it, to above one person, who himself assured
me what it was.

This most curious narrative suggests that in some way Drebel, who
died in London in 1634, had discovered the art of compressing oxygen
and conceived the idea of making it serviceable for freshening the
air in a boat, or other place, contaminated by the respiration of a
number of men for a long time. Indeed the reference made to the
substance by which Drebel purified the atmosphere in his submarine
as "a liquor" suggests that he may possibly have hit upon the secret
of liquid air which late in the nineteenth century caused such a
stir in the United States. Of his possession of some such secret
there can be no doubt whatsoever, for Samuel Pepys refers in his
famous diary to a lawsuit, brought in the King's Courts by the heirs
of Drebel, to secure the secret for their own use. What was the
outcome of the suit or the subsequent history of Drebel's invention
history does not record.

Throughout the next 150 years a large number of inventors and
near-inventors occupied themselves with the problem of the
submarine. Some of these men went no further than to draw plans and
to write out descriptions of what appeared to them to be feasible
submarine boats. Others took one step further, by taking out
patents, but only very few of the submarine engineers of this period
had either the means or the courage to test