6 Mr Watt pn the Origm of his Im]provem&tits 
.necessary for me to consider iii contriving the new en^ne. 
They pointed out that, to avoid useless condensation, the vessel 
in which the steam acted upon the piston ought always to be as 
hot as the steam itself ; that to obtain a proper degree of ex- 
haustion, the steam must be condensed in a separate vessel, 
which might be cooled to as low a degree as was necessary, with- 
out affecting the cylinder ; and that as the air and condensed 
water could not be blown out by the steam as in Newcomen’s, 
they must be extracted by a pump, or some other contrivance ; 
that, in order to prevent the necessity of using water to keep the 
piston air-tight, and also to prevent the air from cooling the cy- 
linder during the descent of the piston, it was necessary to em- 
ploy steam to act upon the piston in place of the atmosphere. 
Lastly, to prevent the cylinder from being cooled by the exter- 
nal air, it was proper to inclose it in a case containing steam, 
and again to inclose that in a case of wood, or of some other 
substance which transmitted heat slowly. 
Although Dr Black’s theory of latent heat did not suggest 
my improvements on the steam-engine, yet the knowledge upon 
various subjects which he was pleased to communicate to me, 
and the correct modes of reasoning, and of making experiments, 
of which he set me the example, certainly conduced very much 
to facilitate the progress of my inventions ; and I still remem- 
ber with respect and gratitude the notice he was pleased to take 
of me when I very little merited it, and which continued through- 
out his life. 
To Dr Robison I am also bound to acknowledge my obliga- 
tions for very much information and occasional assistance in my 
pursuits, and above aU, for his friendship, which ended only 
with his life ; a friendship which induced him, when I was be- 
set with an host of foes, to come to London in the depth of 
winter, and appear as a witness for me in a court of justice, 
whilst labouring under an excessively painful disorder, which 
ultimately deprived him of life. To the remembrance of that 
friendship is principally owing my taking upon myself the office 
of his commentator at my advanced age. 
May I request. Sir, that you and the public will permit that 
age to be my excuse for any errors I may have committed, and 
for any deficiencies in the performance of an office which at no 
