Page:Petty 1851 The Down Survey.djvu/357

 thereby much facilitated the whole processe of surveying." This is not very clear, as there is no other reason to suppose the former surveys were performed by the mode we should now call content-surveying, and it probably refers only to another mode of calculating the areas from the protraction.

Dr. Petty's merit in this respect consisted, not in devising a new mode of surveying, but in availing himself of the means and men he found about him, and organizing a system by which large numbers, "the ministry of about 1000 hands," see p. 295, could be simultaneously employed in different branches of one great survey, instead of separate parties or persons, each completing every branch of a number of small surveys.

The Doctor appears subsequently to have seen the danger of relying on work performed by the needle, as in his Political Anatomy, written some years later, he writes: "The admeasurement of land in Ireland hath hitherto been performed with a circumferencer, with a needle of three two-thirds long, as the most convenient proportion, but twill be henceforth better done by the help of some old geometrical theorems, joyned with the new property of a circle demonstrated by Dr. R. Wood."

Dr. Wood was Master of St. Paul's School in London, and the author of some papers in the early Transactions of the Royal Society, but it is not known what particular property of the circle is here referred to as demonstrated by him.

It is worthy of notice, that about the date at which the Down Survey was performed, there was but little magnetic variation in Ireland. The needle, by computation, pointed due north in Dublin in 1657. This would not afford any peculiar facility for the survey, but might tend to prevent error, both in the field-work and protraction, by careless hands.

In the Record Branch of the Paymaster of Civil Services' Office, there is a book, supposed to be one of the original field-books, and Dr. Petty in his will enumerates original maps and books as among the muniments preserved with his papers. It will not fail to be noticed that in the latter part of these instructions, the Doctor looked beyond the map of forfeitures, and contemplated a more general application of the knowledge to be obtained in a general survey. He had not yet begun to dwell on the studies which afterwards led him to political economy, nor, perhaps, to contemplate the descriptive memoirs which he afterwards began to collect, but the breadth and depth of highways and rivers, their falls and islands, the circumstances of navigable rivers and harbours, course of channel, place of sands and shelves, are all collateral, indeed additional, to the immediate objects of the survey, and indicate a mind aiming at much which was beyond. He alludes to these additions at page 123, where he states that "observing some omissions in his contract, on the states behalfe, he gave out instructions to the respective instruments acting under him, as he humbly conceives, far more large and comprehensive than those contained in his owne contract."

He long afterwards clung to the hope of making a general map. In 1665 we find him petitioning the King for "assistance to finish the Map of Ireland," and in his Political Anatomy (page 341, Dublin Edition), he writes that, "at his own charge, besides those maps of every parish, which by his agreement he delivered into the Surveyor-General's office, he hath