The present position of the science of aërial navigation does not commend itself to Professor Abel as holding out much hope of future success. The recent experiments in France, during which an electrically propelled balloon was made to take more than one short excursion in a predetermined direction, merely prove that electricity can, under exceptionally favourable circumstances, be employed in this new service. But much has been done in making balloons serviceable for purposes of reconnaissance in warfare, the various details, such as making and transporting hydrogen gas in a compressed state to the field of action, having been successfully provided for. Attention was also called in Professor Abel’s address to compressed carbonic acid gas as a convenient source of power. Messrs Krupp, the great cannon-founders, at their extensive works at Essen are using this power for maintaining steel castings under pressure during the solidification of the metal. The earthen mould is closed directly it is filled with metal, after which the compressed gas is admitted to it from a reservoir of liquid carbonic acid, and in this way the space above the molten metal is filled with gas under very high pressure. A tendency to the formation of flaws and cavities, which nearly all metals are subject to—meaning, in the case of railway plant, broken bridges and fractured crank axles—is in this way completely avoided. It is believed that the employment of this gas under pressure—compressed, that is, to the liquid state and stored in iron bottles—has a very wide future before it in many other useful applications. Lastly, the important question of a pure water-supply engaged the professor’s attention, and his opinion on this point will be best given in his own words. ‘I venture,’ he says, ‘to think that our hope for a radical improvement in the water-supply of this great metropolis lies rather in the application of a simple, expeditious, cheap, and effective mode of chemical treatment to supplies from sources now in use, previous to their filtration, than in a complete change of our source of supply.’ It now, therefore, remains for future experimenters to devise some means by which water can be freed from those germs which, under various names, are now said to be responsible for the ills of mankind, and at the same time be left uncontaminated by any foreign matter. The problem seems to be a hard one to solve, but not harder than many which have been successfully conquered by modern science.