XXXIV INTRODUCTION. 



As his work proceeded, the Arab could calmly and skilfully 

 contemplate the effect of the antagonist forces directed to the 

 centre of gravity and the centre of buoyancy of his ship, and 

 survey her equilibrium as it might be permanent or instable ; 

 even though he knew nothing of the fine theory of M. Bouguer, 

 or the laborious calculations of the Swedish Admiral Chapman 

 or of Mr. Atwood, on the hull of the Cuffnells. 



But we have other topics of equal practical importance with 

 the floatation of vessels in this chapter, as for example : 1st. 

 The consideration of a body floating in equilibrio between two 

 fluids which do not mix when the weights of the fluids respec- 

 tively displaced, are together equal to the weight of the solid 

 body which causes the displacement: 2dly. The construction 

 and application of the hydrometer, an instrument generally em- 

 ployed for detecting and measuring the properties and effects of 

 water and other fluids, such as their density, gravity, force and 

 velocity, which depends upon the principles explained and illus- 

 trated in the eighth proposition : 3dly. The hydrostatic balance, 

 an instrument by which we are enabled to measure the specific 

 gravities of bodies with great accuracy and expedition, whether 

 the bodies be in a fluid or a solid state. 



A great many curious facts relating to the equilibrium of 

 floatation could have been here brought under the reader's con- 

 sideration; but these, as well as all popular illustrations of 

 natural philosophy, belong essentially to Somatology, or the 

 properties of matter, a subject which we could not amalgamate 

 with the calculations that illustrate the Mechanics of Fluids. 



The Twelfth Chapter treats of the positions of equilibrium of 

 floating bodies, to determine which, from strict theory, is one of 

 the finest speculations in the whole range of natural philosophy : 

 to ascertain them, as we have done, by computation, involves 

 nothing intricate or repulsive, though the process is both laborious 

 and irksome. To construct them geometrically, demands a know- 

 ledge of principles higher than elementary. And although the 

 geometrical construction may truly represent the position which 

 the body assumes when floating in a state of equilibrium, it is the 

 application of numbers after all, which must determine the true 

 positions. The reason is this ; the specific gravities of the 

 solid and fluid bodies, which are always elements of the in- 



