192 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 
zations were made, of the highest importance on all sides, philosophical, 
economic, and scientific. Carnot, Clausius, Helmholtz, Julius Robert 
Mayer abroad, and the Thomsons, Lord Kelvin and his brother James, 
Rankine, Tait, Joule, Clerk Maxwell, and many others at home, laid 
the foundations on which the splendid structure has been erected. 
That the later.t energy of fuel can be converted into energy of motion 
by means of the steam engine is what we owe to Newcomen and Watt; 
that the kinetic energy of the flywheel can be transformed into elec- 
trical energy was due to Faraday, and to him, too, we are indebted 
for the reconversion of electrical energy into mechanical work; and 
it is this power of work which gives us leisure, and which enables a 
small country like ours to support the population which inhabits it. 
I suppose that it will be generally granted that the Commonwealth 
of Athens attaimed a high-water mark in literature and thought 
which has never yet been surpassed. The reason is not difficult to 
find; a large proportion of its people had ample leisure, due to ample 
means; they had time to think and time to discuss what they thought. 
How was this achieved? The answer is simple: Each Greek free- 
man had on an average at least 5 helots who did his bidding, who 
worked his mines, looked after his farm, and, in short, saved him from 
manual labor. Now, we in Britain are much better off; the popula- 
tion of the British Isles is in round numbers 45 millions; there are 
consumed in our factories at least 50 million tons of coal annually, 
and “‘it is generally agreed that the consumption of coal per indicated 
horsepower per hour is on an average about 5 pounds.” (Royal Com- 
mission on Coal Supplies, Part I.) This gives 7 million horsepower 
per year. How many manpower are equal to a horsepower? I have 
arrived at an estimate thus: A Bhutanese can carry 230 pounds plus 
his own weight, in all 400 pounds, up a hill 4,000 feet high in 8 hours; 
this is equivalent to about one twenty-fifth of a horsepower; 7 million 
horsepower are therefore about 175 million manpower. Taking a 
family as consisting on the average of 5 persons, our 45 millions 
would represent 9 million families; and dividing the total manpower 
by the number of families, we must conclude that each British family 
has, on the average, nearly 20 helots doing his bidding, instead of the 
5 of the Athenian family. We do not appear, however, to have 
gained more leisure thereby, but it is this that makes it possible for 
the British Isles to support the population which it does. 
We have in this world of ours only a limited supply of stored-up 
energy; in the British Isles a very limited one—namely, our coal 
fields. The rate at which this supply is being exhausted has been 
increasing very steadily for the last 40 years, as anyone can prove by 
mapping the data given on page 27, Table D, of the General Report 
of the Royal Commission on Coal Supplies (1906). In 1870, 110 mil- 
lion tons were mined in Great Britain, and ever since the amount has 
