428 Hydrodynamics. 



commencement ; and it is no less evident that it offers a peculiar 

 advantage to engines in which the steam, before being condensed, 

 is employed at a high temperature. Nevertheless, the numerous 

 experiments which have been made within a few years upon 

 engines of this kind, some of which have been accompanied 

 with careful measurements, have not seemed to confirm, so much 

 as might have been expected, the favourable view we have given 

 above ; and if they have taught us a real saving of fuel, when 

 considered with reference to the force actually developed, this 

 saving does not appear to exceed the narrow limits just assigned 

 to the effect of the heat of dilatation. So small an advantage 

 will be far from compensating for all the additional precautions 

 required in such engines, the dangers incurred and the numer- 

 ous causes of waste to which they are liable. It is necessary, in 

 the first place, to make the boilers, cylinder, &c., very strong, 

 that they may resist the expansive force exerted by the steam. 

 It is likewise necessary to give greater perfection to the pistons, 

 and to apply more frequently some lifbricating substance to 

 preserve the contact. Repairs are in consequence often required, 

 and the value of the engine is sensibly diminished on this account. 



Attempts have been made to remove this great inconvenience 

 by an expedient formerly employed to perfect the first invention 

 of Savary. It is this ; the piston, instead of being in immediate 

 contact with the aqueous vapour, which melts and dissolves the 

 grease with which it is impregnated, receives its motion through 

 the intervention of a column of oil, or some other unctuous sub- 

 stance not easily evaporated, on which the steam is made to act 

 by pressure. For this purpose the cylinder in which the piston 

 plays is enclosed in a larger cylinder with which it communi- 

 cates, and which contains the oil. The oil rising and falling 

 continually in the interior cylinder keeps it always lubricated. 

 But although this ingenious arrangement may be sometimes 

 adopted with advantage, it could not be used at high tempera- 

 tures ; for, according to the judicious observation of Mr. Watt, 

 the oil would be decomposed by the dissolving power of the 

 steam. 



It has likewise been proposed to construct high pressure 

 engines having a very great power with very little bulk, by em- 

 ploying small boilers, made so strong as to resist the most 



