Page:Popular Science Monthly Volume 1.djvu/782

762 system. The city of Gratz, containing 80,000 inhabitants, has this system in use in every house, and has thus demonstrated the practicability of using it in large cities. As an illustration of the profit to be derived from human excrement when fairly tested as a fertilizer, Mr. Lepmann refers to the fact that, between the years 1850 and 1864, the price of that obtained from the barracks increased forty-five fold.

Mental Exertions governed by Law.—Prof. Heinrichs read a paper at the Dubuque scientific meeting, "On the Law of Probability as applied to the Determination of Mental Exertions." The following is a summary:

All phenomena in the physical world, exhibited by individuals of a mass subject to certain given influences, are regulated by the so-called law of probability. This has long been practically used by the various insurance companies, which employ millions of dollars a year; however uncertain the health of any given individual, the number of individuals dying each year in a mass of a hundred thousand individuals is perceptibly constant. So also the height of the stature of the individual in a greatly-varying quantity; but the number of individuals in an army having a certain definite height is very nearly constant, and determined by the law of probability. The application of this law of probability to the affairs of the individual man may be studied in the works of Quetelet. By several of our modern chemists the same law has been applied to the various chemical processes. If the laws which regulate mental work and mental phenomena are not radically different from those which we study in the physical world proper, then the law of probability ought to be equally applicable to the mental stature of man, as we long ago have found it to be applicable to the bodily stature of the same. By very careful determination of the relative grade of the individuals composing the large classes which have been instructed in the elements of physics at the State University of Iowa, the author has, during the past three years, had abundant means to test the applicability of the law of probability to mental exertions. The student's standing is determined by adding the numerical values of his credit for oral examination on the subject studied to the grade expressing his daily recitation and his practical work in the laboratory. Since these three quantities are determined independently of one another, and often by different persons (the class being instructed by the professor and two assistants), we have some guarantee against the accumulation of personal errors in this determination. Thus, in a class of sixty-seven students in the elements of physics, the following table shows the observed number of students per hundred who have obtained the standing given, also the calculated number of students who, according to the law of probability, should have obtained the same degree. It will be seen that the two numbers agree very closely:

Dr. Hooker and the Kew Gardens.—The English papers have latterly had much to say of the difficulty between Dr. Hooker, Superintendent of the Kew Botanical Gardens, and Mr. Ayrton, a member of the Government, and Superintendent of Public Works. The Kew Gardens are part of an old royal park situated a few miles out of London, and have been developed to their present great extent and remarkable beauty, as well as in their scientific richness, mainly through the labors of the celebrated botanist Sir William Hooker, and of his son, Dr. Joseph Dalton Hooker, the present director. Without the genius, learning, enthusiasm, and, it may be added, the liberal pecuniary aid of these gentlemen, the Gardens would probably never have been created. They have been called into existence mainly through their agencies, and are now the pride of the nation, and are