in relation to fair and foul Weather; upon several Years Observation of it, it has been proved and adjusted for that purpose by Dr. Robert Hook ; and there have been by him many attempts to improve the Instrument, and render the Minute Divisions on the Scale thereof more sensible. He also judging that it might be of great use at Sea, contrived several ways to make it serviceable on Board of Ship; one of which he explain'd to the Royal Society at their Weekly Meeting in Gresham College , January 2. 1667/8 . Since which time he hath further cultivated the Invention, and some Years ago produced before the said Society , the Instrument I am now to describe, which for its subtilty and usefulness, seemeth to surpass all other performances of the like Nature.
'Till such time as the Author's present Indisposition will give him leave to bestow freely his Thoughts on this Subject upon the Publick, it is the Opinion of the Society, that such an Account be given of this Contrivance, as may render it known, and recommend it to the Mariners use, for which it was principally intended.
The Mercurial Barometer requiring a perpendicular Posture, and the Quick-silver vibrating therein with great Violence upon any Agitation, is therefore uncapable of being used at Sea (tho' it hath lately been contrived to be made portable), so it remain'd to find out some other Principle, wherein the Position of the Instrument was not so indispensably necessary: For this, all those that use the Sea are obliged to the great facility Dr. Hook has always shewn, in applying Philosophical Experiments to their proper uses.
It is about forty Years since, that the Thermometers of Robert de Fluctibus , depending on the Dilatation and Contraction of included Air by Heat and Cold, have been disused, upon discovery that the Airs pressure is unequal; that inequality mixing it self with the Effects of the warmth of the Air in that Instrument. And instead thereof was substituted the seal'd Thermometer , including