It is on thunderbolts as portents of the future that Seneca dwells longest, however.[491] “They give,” he declares, “not signs of this or that event merely, but often announce a whole series of events destined to occur, and that by manifest decrees and ones far clearer than if they were set down in writing.”[492] He will not accept, however, the theory that lightning has such great power that its intervention nullifies any previous and contradictory portents. He insists that divination by other methods is of equal truth, though possibly of minor importance and significance. Next he attempts to explain how the dangers of which we are warned by divination may be averted by prayer, expiation, or sacrifice, and yet the chain of events wrought by destiny not be broken. He maintains that just as we employ the services of doctors to preserve our health, despite any belief we may have in fate, so it is useful to consult a _haruspex_. Then he goes on to speak of various classifications of thunderbolts according to the nature of the warnings or encouragements which they bring. [Sidenote: Ptolemy.] We pass on from Seneca to a later and greater exponent of natural science and divination, Ptolemy, in the following century. He was perhaps born at Ptolemaïs in Egypt but lived at Alexandria. The exact years of his birth and death are unknown, and very little is recorded of his life or personality. The time when he flourished is sufficiently indicated, however, by the fact that his first recorded astronomical observation was in 127 and his last in 151 A. D. Thus most of his work was probably done during the reigns of Hadrian and Antoninus Pius, but he appears to have lived on into the reign of Marcus Aurelius. His strictly scientific style scorns rhetorical devices and literary felicities, and while it is clear and correct, is dry and impersonal.[493] [Sidenote: His two chief works.]