Yugas

Bhishma to Yudhishthira 

'O mighty-armed one, the

1. peace and prosperity of subjects,
2. sufficiency and seasonableness of rain,
3. disease, death and other fears, 

are all dependent on the king.  


I have no doubt also in this. O bull of Bharata's race, that 

1. Krita, 
2. Treta, 
3. Dwapara, and 
4. Kali, as regards their setting in, 

are all dependent on the king's conduct. 

http://www.sacred-texts.com/hin/m12/m12a140.htm