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 when Monsieur Leeuwenhoeck has plainly proved, that he could, with his Glasses, discern Bodies several Millions of Times less than a Grain of Sand. This may be relied upon, because Dr. Hook, who examined what Leeuwenhoeck says of the little Animals which he discerned in Water, of which he tells the most wonderful Things, does, in his Microscopium, attest the Truth of Leeuwenhoeck's Observations.

Besides these which are of more universal Use, several other Instruments have been invented, which have been very serviceable to find out the Properties of Natural Bodies; and by which several Things of very great Moment, utterly unknown to the Ancients, have been detected. As,

1. The Thermometer, invented (a) by Sanctorius, an eminent Physician of Padua. Its immediate Use is, to determine the several Degrees of Heat and Cold; of which our Senses can give us but uncertain Notices; because they do not so much inform us of the State of the Air in it self, as what its Operations are at that Time upon our Bodies. But Sanctorius used only open Vessels, which are of small Use, since Liquors may rise or fall in the Tubes, as well from the Increase or