of heat. In this he used J. P. Joule's paddle-wheel method, though with many improvements, the whole apparatus being on a larger scale and the experiments being conducted over a wider range of temperature. He obtained a result distinctly higher than Joule's final figure. Additionally, he made many valuable observations on the thermodynamics involved, and on the variation of the specific heat of water, which Joule had assumed to be the same at all temperatures. In 1882, before the Physical Society of London, Rowland gave a description of the diffraction gratings, with which his name is specially associated, and which have been of enormous advantage to astronomical spectroscopy. These gratings consist of pieces of metal or glass ruled by means of a diamond point with a very large number of parallel lines, on the extreme accuracy of which their efficiency depends. For their production, therefore, dividing engines of extraordinary trueness and delicacy were required, and in the construction of such machines Rowland's engineering skill brought him conspicuous success. The results of his labors may be found in the elaborate Photographic Map of the Normal Solar Spectrum (1888) and the Table of Solar Wave-Lengths (1898). In his later years, Rowland was engaged in developing a system of multiplex telegraphy. He authored A Plea for Pure Science, in 1883 an important document for the understanding of the relationship between science in university and in commercial contexts in the late nineteenth and early twentieth century. Honors and awards The National Academy of Sciences awarded Rowland the Henry Draper Medal in 1890 for his contributions to astrophysics. He won the Matteucci Medal in 1895. Also, the Henry August Rowland House in Baltimore was designated a National Historic Landmark. See also History of the Tesla coil Hopkinson's law Magnetomotive force Magnetic reluctance X-ray emission spectroscopy X-ray fluorescence Notes References External links Rowland's work and published papers at American Institute of Physics Optical physicists 1848 births 1901 deaths American scientific instrument makers Johns Hopkins University faculty Rensselaer Polytechnic Institute faculty College of Wooster faculty Foreign Members of the Royal Society Phillips Academy alumni Rowland, Henry Augustus 19th-century American physicists Recipients of the Matteucci Medal Presidents of the American Physical Society