376 BALFOUR STEWART. 



remain constant if the heat it received from these objects compensated 

 for that it lost to them. Stewart showed that radiation was not a mere 

 surface phenomenon, — that there was a flow of heat from layer to layer 

 of the particles of a body, — in other words, that there was a flow of 

 heat pervading all matter, and that its direction and amount were deter- 

 mined by molecular conditions, — there being a complete equality be- 

 tween the absorbing and the radiating power of each substance. For 

 his researches on this subject, he was awarded the Rumford Medal 

 by the Royal Society. 



In 1859 Balfour Stewart was appointed Director of the Kew Obser- 

 vatory, and for eleven years devoted himself to meteorology. The 

 account of his labors in this new field can be found in the Reports of 

 the British Association, and cover a great number of subjects, including 

 the testing of thermometers, the perfection of self-recordiug apparatus 

 for the study of the magnetism of the earth, similar apparatus for the 

 study of atmospheric electricity, and the determination of the freezing 

 point of mercury and the melting point of paraffine, with the subsidi- 

 ary researches on the constants of the many forms of meteorological 

 instruments. 



In 1870 he was appointed Professor of Physics in Owens College, 

 Manchester, a position which he held till his death. The character of 

 his mind as an investigator was clearly shown by his advocacy of the 

 laboratory method of instruction in physics. Although he was no 

 longer in vigorous health, having been the victim of a frightful railroad 

 accident, he did not shrink from the serious increase of labor which the 

 laboratory method entails over the lecture and recitation method. His 

 treatise on Practical Physics is one of the best laboratory treatises in 

 physics, and forcibly illustrates the peculiar quality of the author's 

 mind, which was marked by a philosophical breadth in the choice of 

 methods to cultivate the scientific instinct. 



By the publication of elementary treatises on Heat, on Practical 

 Physics, on Elementary Physics, and on the Conservation of En- 

 ergy, Stewart contributed largely to the cause of scientific education. 

 Among these treatises, that on Heat easily takes the first place from 

 a scientific point of view, and can be entitled a classic. It is prob- 

 able that the general reader of science first gained his ideas of the 

 great generalization of the conservation of energy from Stewart's 

 simple exposition of the subject. He was also a frequent contributor 

 to " Nature," and other scientific periodicals, and he wrote an article 

 on Terrestrial Magnetism for the Encyclopaedia Britannica. He also 

 wrote, in conjunction with De la Rue and Loewy, a series of papers 



