The Correspodence between S. M. Kozhevnikov and J. P. Den Hartog as a Source for the Study of Scientific Collaboration between the Mechanicians (to the 110th Anniversary of Sergei Kozhevnikov's birth) - M. O. Pryn; O. V. Pryn; S. I. Degtyarev (Sumy Historical and Archival Journal, 2016)