DA VIES GILBERT'S OPINION. 3S9 



then went on to state that the great defect in all steam-engines 

 seemed to be the loss by condensation of all the heat rendered 

 latent in the conversion of water into steam. That high-pressure 

 engines owed their advantages mainly to a reduction of the 

 relative importance of this latent heat. That I had long 

 wished to see the plan of a differential engine tried, in which 

 the temperatures, and consequently elasticities, of the fluid 

 might be varied on the opposite sides of the piston without con- 

 densation ; that the engine you had now constructed promised 

 to effect that object, and that in the event of its succeeding at 

 all, although it might not be applicable to the driving water out 

 of mines, yet that for steam-vessels and for steam-carriages its 

 obvious advantages would be of the greatest importance ; and I 

 ended by saying that although it was clearly iuipossible for me 

 to ensure the success of any plan till it had been actually proved 

 by experiment, yet judging theoretically, and also from the 

 imperfect trial exhibited on the Thames, I thought it well 

 worthy of being favoured. 



"Your plan unquestionably must be to appoint some one 

 with you, as Mr. Watt did Mr. Boulton, and I certainly think it 

 a very fair speculation for any such person as Mr. Boulton to 

 undertake. 



" It is impossible for me to point out any individual, as never 

 having had the slightest motive with such or with manufacturers 

 in any part of my life, I am entirely unacquainted with mercan- 

 tile concerns. I cannot, however, but conjecture that you should 

 make a fair and full estimate of what would be the expense of 

 making a decisive experiment on a scale sufficiently large to 

 remove all doubt ; and that your proposal should be, that any- 

 one wishing to incur that expense should, in the event of success, 

 be entitled to a certain share of your patent; on such conditions 

 some one of property may perhaps be found who would undertake 

 the risk, and if the experiment proved successful, he \vould be 

 sure to use every exertion afterwards for his own sake. With 

 every wish for your success, 



" Believe me, yours very faithfully, 



" DAVIES GILBERT." 

 VOL. ir. 2 D 



