The service of augurs was most needed in establishing the terrestrial templum[624]—a carefully marked out, oriented spot which the magistrate occupied while performing the rite.[625] Whereas the commander of an army generally made use of chicken auspices (signa ex tripudiis), which did not require their assistance,[626] they were doubtless always called upon to institute templa in or near the city.[627] For the exercise of their art they divided the world, so far as known to them, into augural districts. The central district was the city, limited by the pomerium,[628] beyond which, probably extending to the first milestone,[629] lay a zone termed ager effatus,[630] whose boundaries were marked by cippi.[631] The rest of the world within their sphere of knowledge they divided into ager Romanus, which in its larger sense included the two districts above mentioned, Gabinus, peregrinus, hosticus, and incertus.[632] For the comitia the two inner regions were alone important: (1) the auspication of assemblies held in the city had to be performed within the pomerium; (2) as often as the magistrate in passing from the city to the Campus Martius to hold the comitia centuriata crossed the pomerium,[633] or more strictly the brook Petronia,[634] he was obliged to take the special auspices of crossing. Beyond the ager effatus assemblies were not ordinarily held. Originally the most common form of divination must have been the watching of the flight of birds, for it is from this ceremony that the word auspicium is derived.[635] Legend accordingly asserts that Romulus founded the city on the Palatine under the auspices of twelve vultures.[636] Before the end of the republic, however, all other forms of public auspicia impetrativa in the city had given way to the caelestia, especially the lightning and thunder.[637] The reason is that the heavenly signs could be most easily understood and carried greatest weight; whereas other auspices had to be held for each individual act, the celestial omens of the morning served the magistrate for all his undertakings during the entire day.[638] The effect of heavenly signs on assemblies of the people, however, was peculiar. Not only were comitia and contiones interrupted by storms;[639] not only was it impious to hold an assembly while it was lightning or thundering,[640] but even while the magistrate was auspicating at daybreak, if a flash of lightning appeared on the left—a sign favorable for every other undertaking—he dared not hold the assembly on that day.[641] Some favorable comitial sign the magistrate was supposed to perceive,[642] but what it was we do not know.