Complete the following sentence: During Frederick's rule Copenhagen was struck by two disasters: the plague of 1711, and the
️<unk>️<unk>️<unk>️<unk>️<unk>️<unk>️<unk>️ 1710) to introduce the Gregorian calendar in Denmark-Norway in 1700, the astronomer's observations and calculations were among the treasures lost to the fire.