Tuesday, November 10, 2009

Rockets and Angles

In the attack by Jack “the Coiner” Shaftoe on the Tower of London, on 23 April 1714, his accomplices used several rockets (pages 239 ff of The System of the World). One dismantled rocket was carried to the top of The [Fire] Monument by Jack’s sons. On page 241, its head was described as “... a great long helmet-shaped object ...”, and the guide stick was camouflaged as a pilgrim staff. On page 248, the preparation for firing was described as “... they had lashed the pilgrim-staff to the rocket-head, and leaned it against the railing, aimed in the general direction of St. Mary-at-Hill.”

Another rocket was delivered preassembled to All Hallows Church (page 247). “He diverted his glass a few arc-seconds down into the adjoining churchyard, where the funeral had taken a macabre turn; the lid of the coffin had been tossed aside to reveal a helmet-shaped object with a long stick projecting from its base.”

They seemed to be remarkably reliable and accurate (pages 282-284). None of them missed fire, and the rockets from The Monument and All Hallows Church both passed directly over their intended targets. The first two rockets from the barge on the Thames went short or wide, but the third hit directly on the roof of the White Tower. (That was the only launch site that had spares.)

One ought to wonder where Jack obtained these rockets. Rockets had been used in both Asia and Europe by that time, as display fireworks and as weapons. However, according to my Encyclopaedia Britannica, 1714 fell in the middle of a 100-year period of only sporadic use of rockets in wars. Most rockets of that time had heads made of organic materials (e.g. paper, cloth, or wood), which could not withstand high internal pressure. The descriptions by Stephenson unfortunately do not mention the material, which might help identify the source.

The first thoroughly developed rockets were probably those of William Congreve, but they came later. (See Wikipedia.) They were based on the iron-headed rockets which had been used against the British in the Mysore wars (1792 and 1799). Congreve rockets were first employed against the French in 1806, and notably in the War of 1812. The U.S. National Anthem mentions “the rockets’ red glare” over Fort McHenry, near Baltimore, in 1814. Those were Congreve rockets, and their shape roughly matched Stephenson’s description. If only Jack’s rockets had been Congreve rockets, this would have afforded another example of the continuity which is a notable feature of these four novels. As Lawrence P. Waterhouse was playing the glockenspiel part of the National Anthem on the deck of USS Nevada at Pearl Harbor on December 7, 1941 (Cryptonomicon, page 77), he took advantage of the fact that “... The Star Spangled Banner is much easier to ding than to sing.”

However, Congreve rockets were rather inaccurate. Even launched from a standard frame, they could start off at a wide angle from the intended direction. They could also change direction in mid-flight. This erratic behavior actually increased their effectiveness as weapons of terror against exposed enemy forces. Congreve advised that they should be launched in volleys of 20 to 50. Surely the rockets of a century earlier were even more erratic than that.

Stephenson seems to be enamored of the arc-second as a unit of angle. To find the actual angle through which Jack depressed his spyglass in shifting from the roof of All Hallows Church to the adjacent graveyard, we need to estimate some lengths. The four stated distances along a somewhat zig-zag line from The Monument to All Hallows add up to about 1600 feet. However, from my only map of London with a scale (Baedecker’s), that direct distance seems to be about 1200 feet. I can only guess the roof of that church to have been about 40 feet high. Thus the vertical angle as seen from The Monument was about 1/30 radian, or roughly seven thousand arc-seconds, or two degrees. The mere tremor in Jack’s arms, due to his pulse-beat, surely deflected the spyglass through an angle larger than “a few arc-seconds”.

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