298 POPULAR SCIENCE MONTHLY. 



of the way in which diseases are transmitted. With this knowledge it 

 is possible to prevent the diseases. The great fact that the death rate 

 is decreasing stands out prominently and proclaims to humanity the 

 importance of scientific investigation. It is, however, to be noted in 

 this connection that the decrease in the death rate compensates to some 

 extent for the decrease in the birth rate, and that, if an increase in 

 population is a thing to be desired, the investigations in the field of 

 sanitary science are contributing to this result. 



The development of the human race is dependent not alone upon 

 a supply of food but upon a supply of energy in available forms. Heat 

 and mechanical energy are absolutely essential to man. The chief 

 source of the energy that comes into play is fuel. We are primarily 

 dependent upon the coal supply for the continuation of the activities 

 of man. Without this, unless something is to take its place, man is 

 doomed. Statistics in regard to the coal supply and the rate at which 

 it is being used have so frequently been presented by those who have 

 special knowledge of this subject that I need not trouble you with 

 them now. The only object in referring to it is to show that, unless 

 by means of scientific investigation man is taught new methods of 

 rendering the world's store of energy available for the production of 

 heat and of motion, the age of the human race is measured by the 

 extent of the supply of coal and other forms of fuel. By other forms 

 of fuel I mean, of course, wood and oil. Plainly, as the demand for 

 land for the production of foodstuffs increases, the amount available 

 for the production of wood must decrease, so that wood need not be 

 taken into account for the future. In regard to oil, our knowledge is 

 not sufficient to enable us to make predictions of any value. If one of 

 the theories now held in regard to the source of petroleum should prove 

 to be correct, the world would find much consolation in it. According 

 to this theory petroleum is not likely to be exhausted, for it is con- 

 stantly being formed by the action of water upon carbides that in all 

 probability exist in practically unlimited quantity in the interior of 

 the earth. If this be true, then the problem of supplying energy may 

 be reduced to one of transportation of oil. But given a supply of oil 

 and, of course, the problem of transportation is solved. 



What are the other practical sources of energy? The most impor- 

 tant is the fall of water. This is being utilized more and more year 

 by year since the methods of producing electric currents by means of 

 the dynamo have been worked out. There is plainly much to be learned 

 before the energy made available in the immediate neighborhood of 

 the waterfall can be transported long distances economically, but ad- 

 vances are being made in this line, and already factories that have 

 hitherto been dependent upon coal are making use of the energy de- 

 rived from waterfalls. The more rapidly these advances take place 



