d reminder of the fact that Watt
was the discoverer of the practically constant and unvarying amount of
heat in steam, whatever the pressure, is the following by Mr. Lauder, a
graduate of Glasgow University and pupil of Lord Kelvin, taken from
"Watt's Discoveries of the Properties of Steam."

It is well to distinguish between the two things, Discovery and
Invention. The title of Watt the Inventor is world-wide, and is
so just and striking that there is none to gainsay. But it is
only to the few that dive deeper that Watt the Discoverer is
known. When his mind became directed to the possibilities of the
power of steam, he, following his natural bent, began to
investigate its properties. The mere inventor would have been
content with what was already known, and utilised such
knowledge, as Newcomen had done in his engine. Watt might have
invented the separate condenser and ranked as a great inventor,
but the spirit of enquiry was in possession of him, and he had
to find out all he could about the _nature_ of steam.

His first discovery was that of latent heat. When communicating
this to Professor Black he found that his friend had anticipated
him, and had been teaching it in lectures to his students for
some years past. His next step was the discovery of the _total_
heat of steam, and that this remains practically constant at all
pressures. Black's fame rests upon his theory of latent heat;
Watt's fame as the discoverer of the total heat of steam should
be equally great, and would be no doubt had his role of inventor
not overshadowed all his work.

This part of Watt's work has been so little known that it is
almost imperative to-day to give some idea of it to the general
reader. Suppose you take a flask, such as olive oil is often
sold in, and fill with cold water. Set it over a lighted lamp,
put a thermometer in the water, and the temperature will be
observed to rise steadily till it reaches 212 deg., where it
remains, the water boils, and steam is produced freely. Now draw
the thermometer out of the water, but leaving it still in the
steam. It remains steady at the same point--212 deg. Now it
requires quite a long time and a large amount of heat to convert
all the water into steam. As the steam goes off at the same
temperature as the water, it is evident a quantity of heat has
escape