Agriculture on a large scale:
Crops had to be planted and harvested in the right season. Floods were a recurrent phenomenon in the river valleys, for which it was essential to be prepared beforehand. The observation of sun and stars to fix the length of a year became so precise that already in 2700 B.C., the Egyptians were able to fix it at 365.2422 days! Sumerians and their successors in Mesopotamia adjusted t'he solar (sun-based) and lunar (moon-based) calendars through accurate observations. They invented the sexagesimal system of 360 degrees in a circle (near enough to the days in a year), 60 minutes in an hour, 60 seconds in a minute. The exercises were carried out using mathematical tables and ied to algebra and arithmetic of the later epochs. Another occupation that came to be very prestigious with the growth of cities was that of medicine. Although the practice of medicine was limited to treating wounds, dislocations, fractures etc., the practitioners could successfully diagnose many diseases. They could compare cases with one another, notice different diseases and record them. From such descriptions, orally passed on to later generations, arose the sciences of anatomy and physiology. Practitioners of medicine also had the knowledge of plants and mineral substances to prepare drugs for various diseases. They grew plants and herbs for this purpose. It is from this source that the science of botany arose later. The basis for chemistry was laid in the observations and practices of jewellers, metal- workers and potters. They knew about at least nine chemical elements-gold, silver, copper, tin, lead, mercury, iron, sulphur and carbon, and also about a variety of dry and liquid reagents. The process of smelting ores, of purifying metals, of colouring them, of adding enamel%-all involve complex chemical reactions that were learnt by many trials and experiments. However, chemistry never rose to the rank of a recognised science in the 'Bronze Age.