ON THE HISTORY OF HYDRAULICS, &c. 285 



after maintained a contrary opinion in the commentaries of Bologna,* and 

 Count Rumford has very satisfactorily shown the insufficiency of the 

 agents considered by Robins, although he has been unsuccessful in at- 

 tempting to deduce the whole force from the elasticity of aqueous vapour 

 alone. 



The theory of practical hydraulics, as affected by friction, may be con- 

 sidered as having been begun and completed by the highly meritorious 

 labours of the Chevalier du Buat. He had some assistance in expressing 

 the results of his experiments by means of general rules or formulae, and 

 these, although they agree sufficiently well with the experiments, have not 

 always been reduced to the simplest and most convenient forms ; nor have 

 they been much improved either by Langsdorf or Eytelwein in Germany, 

 or by Robison in this country, who have gone over nearly the same ground 

 with each other, and have shown the way in which the results of Buat's 

 investigations may be applied to a variety ofoases, which occur in hydrau- 

 lic architecture. 



One of the latest inventions which require to be mentioned in speaking 

 of the history of pneumatics, is that of the aerostatic globe or air balloon. 

 The suggestions of Lohmeier,t of Albertus, and of Wilkins,^ respecting 

 the various modes of passing through the air, had long remained disre- 

 garded as idle speculations ; and Rosnier, who, in the 17th century, 

 descended obliquely over some houses, by means of wings, was wholly 

 unable to employ them in ascending. Dr. Black had exhibited in his 

 lectures a bladder filled with hydrogen gas, and floating in the air by 

 means of its smaller specific gravity, many years before Montgolfier con- 

 ceived the idea of applying a similar machine to the elevation of human 

 beings into the aerial regions. It was in 1783 that this project was first 

 executed, and persons of a warm imagination were disposed to believe that 

 the discovery would be of great importance to the convenience of mankind. || 

 But if we coolly consider the magnitude of the force with which the wind 

 unavoidably impels a surface so large as that of a balloon, we shall be con- 

 vinced of the absolute impossibility of counteracting it, in such a manner, 

 as to direct the balloon in any course, materially different from that of the 

 wind which happens to blow. With this limitation, the invention may 

 still in some cases be capable of utility, wherever we are only desirous 

 of ascending to a great height, without regarding the place in which we 

 are to descend : or where we wish to attain only a height so moderate that 

 the machine may be kept by ropes in the situation which is desired. In 

 France the balloon has lately been employed with considerable success as 

 a meteorological observatory; Mr. Biot and Mr. Gay Lussac having 

 ascended to a height of above four miles, for the laudable purpose of ascer- 

 taining some facts relating to the constitution of the atmosphere, and to 

 the magnetic properties of the earth. 



* iii. 92 ; iv. 106. f De Artificio Navigandi per Aerem, 1676. 



J Mathematical Magic, 1680. Hooke, Ph. Coll. No. 1, p. 15. 

 '|| Montgolfier, Discours sur TAerostate, Paris, 1784. P. de Rozier, Premiere 

 Experience de la Montgolfiere, 4to, Paris, 1784. 



