The relative relationships “yield – evapotranspiration” were used long time ago. The well known linear relationship yi = 1 – ky (1 – ei), where yi is relative yield, ky – yield response factor and ei – relative evapotranspiration was proposed. It’s usually assumed that ky is constant for a given crop and climatic conditions. It was found, however, that ky for late variety of maize H 708 varied through the study years (1984–1990) in the Plovdiv region (South Bulgaria, altitude 150 m). During the dry years it was significantly higher than in the medium and humid years. The range of ky for maize in this location was 1.12–1.90, the average value being 1.50. The climate in the Sofia region (the ex-perimental field of Chelopechene, altitude 550 m) is comparatively more humid. The two regions approximately outlined the boundaries of the appropriate economical conditions for grain maize pro-duction. The experiments in the Sofia region were carried out in the years 1994–2000. The seven years results for mean variety maize showed that the relationships “yield – evapotranspiration” and, respectively, ky varied at these climatic conditions too. The highest ky value was 1.41 for the driest year (2000) and the lowest value – 1.05 for the most wet years (1995, 1999). The value of ky for av-erage years was 1.21. The yield response factor ky is of more significance when the relative evapotranspiration is less than 0.7–0.8. Thus, the extreme or the average values of ky could be used for the corresponding climatic regions. The relationships between ky and relative yield were estab-lished without considering irrigation.
The letter of Muscovy ruler Boris Godunov which is held in the Library of Kórnik, is the earliest historical document that testifies about the relations between the government of Muscovy and Ukrainian Cossacks since the end of the sixteenth century. In the article are reviewed the historical context of such contacts and their long term consequences. At that time, led by Krzysztof Kosynskyi, Cossacks entered into conflict with powers of Commonwealth. Godunov informed Cossacks that his government invites them for military service. It was the first time when Cossacks obtained the opportunity to make a choice between the service to the Polish King or Muscovy Tzar. During the next a half of the century, Ukrainian Cossacks transformed into a powerful military corporation, which influenced on the balance of powers in international relations of the region. Later on, when in the consequence of the war with Poland, Cossacks switched themselves to the side of Muscovy in 1654, this change provided a foundation for the hegemony of Muscovy in East Europe.