described in a paper in the American Journal of Science and Arts, entitled "A New Apparatus for the Determination of Carbonic Acid," which was republished in Germany. His second paper was on the estimation of weights of very small portions of matter, in which he showed that, by the deflections of fine glass fibres, we can weigh a particle of matter as small as the 1000 of a milligramme. While at the Lehigh University, he designed and superintended the erection of an astronomical observatory, put together and adjusted the instruments, and made a series of observations on the planet Jupiter, which were republished in England. He was in charge of the party sent by the United States Government to observe the eclipse of the sun at Burlington, Iowa, August 7, 1869, and took forty-one perfect photographs of the eclipse with exposures lasting only the 500 of a second.
While at the Lehigh University, Prof. Mayer contributed to the American Journal of Science and Arts and the Journal of the Franklin Institute various original contributions on the "Solar Protuberances," "Spectral Analysis of the Stars," "Physical Constitution of the Sun," "Electro-Magnetism," "Electric Conductivities," "On the Alleged Electro-Tonic State," "Magnetic Declination in Connection with the Aurora," "Photographing the Magnetic Spectra;" and at the Salem meeting of the American Association he read an instructive paper "On the Thermo-Dynamics of Waterfalls." He had taken the temperatures of the water at Trenton and Niagara before it leaps the cataract and after it strikes below. According to theory, when the falling motion is arrested, it is converted into heat, and should be shown in a rising temperature below. The observations indicated that the effects of evaporation and contact of the divided water with the air were greater than the impact in changing the temperature of the fallen water, so that it may be actually colder below than above. But on days when the air is saturated with moisture, and the temperatures of the water and air are about the same, results were obtained which show that the warming of the falling water conforms to Joule's law of the conversion of motion into heat.
Since entering upon his duties at the Stevens Institute of Technology, notwithstanding the labor of lecturing and teaching which the position involves, Prof. Mayer has conducted elaborate investigations in various branches of physics, which have given to science many new and important results. Among these may be mentioned researches in magnetism, heat, and especially "On the Effects of Magnetization in changing the Dimensions of Iron and Steel Bars," and "On the Isothermals of the Solar Disk." We cannot here give the particulars of these interesting inquiries, but must refer the reader to the memoirs in the scientific journals.
The line of investigation, however, to which Prof. Mayer has mainly devoted himself within the last few years is that of acoustics