There is a certain analogy, constancy, and uniformity in the phenomena or appearances of nature, which are a foundation for general rules: and these are a grammar for the understanding of nature, or that series of effects in the visible world whereby we are enabled to foresee what will come to pass in the natural course of things. Plotinus observes, in his third Ennead, that the art of presaging is in some sort the reading of natural letters denoting order, and that so far forth as analogy obtains in the universe, there may be vaticination. And in reality he that foretells the motions of the planets, or the effects of medicines, or the result of chemical or mechanical experiments, may be said to do it by natural vaticination.
By the way, although this is just a guess, I think Berkeley may have had this passage from the Iliad in mind when he coined the term 'Siris' by Anglicizing a Greek word for cord or chain:
Hearken unto me, all ye gods and goddesses, that I may speak what the heart in my breast biddeth me. Let not any goddess nor yet any god essay this thing, to thwart my word, but do ye all alike assent thereto, that with all speed I may bring these deeds to pass. Whomsoever I shall mark minded apart from the gods to go and bear aid either to Trojans or Danaans, smitten in no seemly wise shall he come back to Olympus, or I shall take and hurl him into murky Tartarus, far, far away, where is the deepest gulf beneath the earth, the gates whereof are of iron and the threshold of bronze, as far beneath Hades as heaven is above earth: then shall ye know how far the mightiest am I of all gods. Nay, come, make trial, ye gods, that ye all may know. Make ye fast from heaven a chain of gold, and lay ye hold thereof, all ye gods and all goddesses; yet could ye not drag to earth from out of heaven Zeus the counsellor most high, not though ye laboured sore. But whenso I were minded to draw of a ready heart, then with earth itself should I draw you and with sea withal; and the rope should I thereafter bind about a peak of Olympus and all those things should hang in space. By so much am I above gods and above men.
That's from Iliad 8 (you can see the Greek at the same site). Zeus, of course, is the speaker. Plato mentions in the passage in Theaetetus (153c). Another place where seirais are mentioned is Josephus's Antiquities (Bk. III, section 170), where the word is applied to the chains on the high priest's garment; but I don't know if Berkeley would have been thinking of this. The Homeric passage is certainly close to what Berkeley intended. (For which, see his poem On Tar.)