producing a double Distillation. 589 
The water distilled over is collected in the receiver, which is 
kept cool for that purpose. 
An apparatus of this kind I had constructed chiefly for the 
purpose of explaining the theory of latent heat, or of the 
capacity of bodies for heat in different states ; but it is possible 
that it may also be of some further practical utility, whenever 
it is of consequence to economise the consumption of fuel. 
When water is deficient on board of ship, it has been in some 
degree supplied by distillation from the ship's boiler, and if 
the steam from the boiler had been made to pass through the 
apparatus just described, the quantity would be nearly doubled. 
By an experiment which I had made some time ago, about 
three fourths of the quantity obtained by the first distillation 
were added by the second. But I believe a larger proportion 
may be procured when the second distilling vessel is duly 
coated with flannel, or some light substance, to retain the 
heat. 
Though salt water does not boil at so low a degree of heat 
as fresh water, yet upon trial with sea w’ater the difference 
was found to be quite insignificant, compared with tliat of the 
steam formed under the usual pressure, and in vacuo ; and did 
not sensibly affect the result of the process. The only doubt 
as to the propriety of taking such a vessel to sea, would arise 
from the degree of danger which there is of experiencing a 
want of fresh water. This probably, I apprehend, is not great ; 
but on the other hand, there is the important object of saving 
the lives of the people in the ship, whenever such deficiency 
is experienced. 
MDCCCXIV. 
