Plutarch, Numa
Bernadotte Perrin, Ed.

2. For thirty-seven years, now, Rome had been built and Romulus had been its king; and on the fifth of the month of July, which day they now call the Capratine Nones, Romulus was offering a public sacrifice outside the city at the so-called Goat's Marsh, in the presence of the senate and most of the people. [2] Suddenly there was a great commotion in the air, and a cloud descended upon the earth bringing with it blasts of wind and rain. The throng of common folk were terrified and fled in all directions, but Romulus disappeared, and was never found again either alive or dead. Upon this a grievous suspicion attached itself to the patricians, and an accusing story was current among the people to the effect that they had long been weary of kingly rule, and desired to transfer the power to themselves, and had therefore made away with the king. And indeed it had been noticed for some time that he treated them with greater harshness and arrogance. [3] This suspicion the patricians sought to remove by ascribing divine honours to Romulus, on the ground that he was not dead, but blessed with a better lot. And Proculus, a man of eminence, took oath that he had seen Romulus ascending to heaven in full armour, and had heard his voice commanding that he be called Quirinus.¹

[4] The city was now beset with fresh disturbance and faction over the king to be appointed in his stead, for the new comers were not yet altogether blended with the original citizens, but the commonalty was still like a surging sea, and the patricians full of jealousy towards one another on account of their different nationalities. It is indeed true that it was the pleasure of all to have a king, but they wrangled and quarrelled, not only about the man who should be their leader, but also about the tribe which should furnish him. [5] For those who had built the city with Romulus at the outset thought it intolerable that the Sabines, after getting a share in the city and its territory, should insist on ruling those who had received them into such privileges; and the Sabines, since on the death of their king Tatius they had raised no faction against Romulus, but suffered him to rule alone, had a reasonable ground for demanding that now the ruler should come from them. They would not admit that they had added themselves as inferiors to superiors, but held rather that their addition had brought the strength of numbers and advanced both parties alike to the dignity of a city. On these questions, then, they were divided into factions.

[6] But in order that their factions might not produce utter confusion from the absence of all authority, now that the administration of affairs was suspended, it was arranged by the senators, who were one hundred and fifty in number,² that each of them in his turn should assume the insignia of royalty, make the customary sacrifices to the gods, and transact public business, for the space of six hours by day and six hours by night. [7] This distribution of times seemed well adapted to secure equality between the two factions, and the transfer of power likely to remove all jealousy on the part of the people, when they saw the same man, in the course of a single day and night, become king and then a private citizen again. This form of government the Romans call ‘interregnum.’