I shall take this opportunity of remarking, that those small glasses hermetically sealed and containing a drop of water, which when placed on hot coals burst with a loud report, and therefore are called in German _knallgläser_, fulminating glasses, were known before 1665. Hooke speaks of them in his Micrographia[580] printed in that year, and they were mentioned by Reyher in 1669, in his Dissertation already quoted. In Germany they are made chiefly by Nuremberg artists; one of the most celebrated of whom was Michael Sigismund Hack. He learnt the art of glass-blowing in England, and in 1672 returned to Nuremberg, where he was born in 1643[581]. FOOTNOTES [573] It is not always necessary that the water should be cold; these drops will be formed also in warm water, as well as in every other fluid, and even in melted wax. See Redi’s experiments in Miscellan. Naturæ Curios. anni secundi, 1671, p. 426. They succeed best with green glass, yet I have in my possession some of white glass, which in friability are not inferior to those of green. [574] The navel, in German _nabel_, is that piece of glass which remains adhering to the pipe when any article has been blown, and which the workman must rub off. These navels, however, are seldom in so fluid a state as to form drops. [575] Journal des Voyages de M. Monconys, Lyon, 1666, 4to, ii. p. 162. [576] Commentarius de rebus ad eum pertinentibus, Lips. 1719. [577] Historia Naturalis. Edit. secunda, Londini 1680, 4to, p. 37. [578] In his Observations on Neri Ars Vitraria, Amstel. 1668, 12mo. [579] This is said, for example, by Grainger in his Biographical History of England. London, 1769, vol. ii. part 2, p. 407. [580] This book was only once printed, but the title-page has the date 1667. See Biographia Britannica, iv. p. 2654. [581] Doppelmayer, p. 276. FIRE-ENGINES. The invention of pumps I shall leave to those who undertake to write the history of hydraulics, and here only remark that, on the testimony of Vitruvius[582], it is in general ascribed to Ctesibius, on which account they are called _machinæ Ctesibicæ_; and that Ctesibius lived at Alexandria in the time of Ptolemy Philadelphus and Ptolemy Euergetes I., consequently two centuries before the Christian æra. My present object extends no further than to state what I know in regard to the question, At what time were these machines first employed for extinguishing fires?