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First Bowditch displays impressive knowledge

Jan 1, 2003

It is difficult, in this age of electronic charts, to truly appreciate the depth of knowledge of the navigation scientists of the 18th and 19th centuries. We tend to think we have a corner on knowledge, and that all that ancient stuff was technologically primitive and, therefore, represented less advanced thinking. Examining a copy of the first edition of The New American Practical Navigator by Nathaniel Bowditch, gives one a new appreciation of his accomplishments. He and the other scientists of his day used something much more remarkable than computer power. They used brain power. Let's take a tour of a copy of the first edition of "Bowditch" and see why this is true.

First, we need to look at the historical perspective. Chronometers had been invented in the 1760s, and spherical trigonometry promised a solution to the longitude problem as soon as they became commonly available. Unfortunately, the price of the first generation of chronometers was far too high for a ship owner to absorb, and, until a market developed, they were prohibitively expensive. So the common navigator relied on latitude sailing and DR for most voyages. A captain with the requisite mathematical skill could purchase two books to help him find his longitude. The first was Elements of Navigation by John Robertson, published in England in 1750. The problem with this book was that it was incomplete and did not contain all the required tables to complete the complicated lunar distance calculations necessary for a longitude line. The second work required was Requisite Tables, published by the Astronomer Royal in England, Nevil Maskelyne. This contained, in large part, the tables missing from Robertson's work. Armed with both, the navigator had some hope, after long and tedious calculations, of finding his longitude within a few miles. But there were still problems.

Enter John Hamilton Moore. He decided to compile a navigational text containing all the required tables in one volume, along with explanations. Using Robertson's and Maskelyne's tables, and adding text to explain their use in practical navigational problems, he published The Practical Navigator, both in England and America. The American version was known as The American Practical Navigator. It was a very successful work. Unfortunately, he copied all the same mistakes which his sources had made, inadvertently adding several of his own. One of these resulted in the loss of at least one ship.

Then came Nathaniel Bowditch. There was probably not a more meticulous nor intelligent man in America at that time, and both qualities were absolutely essential for the work he was to do. A good summary of his life is found in the front of the modern American Practical Navigator. Nathaniel Bowditch set out to repair the mistakes in Moore's work. He started by recomputing Moore's tables. We can only marvel at the mind which did those thousands upon thousands of calculations, and the hand which painstakingly wrote them out in longhand. What endless hours, it must have taken.

The formulas for producing the tables were well known. The point of the tables was to provide the navigator with a way to look up the answers he needed instead of having to do the tedious calculations for each sight. But the only way for Bowditch to ensure the accuracy of the original tables was to recompute them all. And there were no calculators in those days, not even mechanical ones. During this effort, Bowditch found some 8,000 errors in Moore's tables. While he was at it, he corrected some 2,000 errors in Robertson's original Requisite Tables. More than just a few tables were in involved. In fact, the first edition of Bowditch's The New American Practical Navigator contained 29 tables. Below is a verbatim listing of the tables as published in the original work:· Difference of Latitude and Departure for Points ·Ditto for Degrees · Meridional Parts · Sun's Declination · For Reducing the Sun's Declination · Sun's Right Ascension · Amplitudes · Right Ascension and Declination of the fixed stars, with precepts for using the tables · Sun's Rising and setting, with precepts·For finding the distance of Terrestrial Objects at sea, explained in page 205, Prob. VIII. and IX·Proportional Parts · Refraction of the heavenly bodies· Dip of the horizon· Sun's Parallax in altitude· Augmentation of the Moon's semi-diameter · Dip for different heights and distances· Correction of the Moon's altitude for parallax and refraction, explained page 181· Table for finding the third correction of a Lunar Observation· For turning degrees and minutes into time, and the contrary· For finding Latitude by two altitudes of the sun· Natural sines and Cosines · Proportional Logarithms· Log. Sines, Tangents, &c. to points and quarter points· Logarithms of Numbers · Artificial or Logarithmic Sines, Tangents, &c.· To find the time of the Moon's passing the meridian, explained page 154· Correction of the Moon's Altitude for parallax and refraction, explained page 155· To find the Moon's Declination, explained page 154· To find the Sun's Right Ascension, with precepts for use of it

That was just the tables section of The New American Practical Navigator. Preceding these was a text section of 247 pages (of very small print) explaining virtually every known detail of the navigator's craft, much of which is still practiced today. Included are: · Geometry · Trigonometry · Use and calculation of logarithms· Astronomy· Explanations of the sailings· Meridional parts· Construction and use of Mercator charts· Complete directions on the use of the quadrant or sextant· Compass variation and measurement and calculation of amplitudes and azimuths· Latitudes by meridian altitudes of the sun, moon, and planets· Finding apparent time at sea· Longitude by lunar distances· Calculation of the altitude of the sun, moon, and stars· Finding longitude by eclipses of Jupiter's moons · Finding longitude by eclipses of the moon and by a variation chart· Mensuration· Gauging· Hydrographic surveying of coasts and harbors· Tides·Winds· Currents

There was much more in addition to that listed above, comprising 75 different chapters.

To appreciate the amount of work Bowditch accomplished, the following quotes from the Preface to the first edition, written by Bowditch himself, are reproduced below as found in the original edition. They are typical of summaries of all the tables.

"Table V. contains the declination of the sun, which was compared with the Nautical Almanac for the years 1801, 1802, 1803, and 1804, and marked to the nearest minute. This table is one of the most important in this collection, because the latitude is generally determined by it: it was therefore a very criminal inattention of Moore, in publishing it so incorrectly in most of the late editions of his work; for by reckoning the year 1800 as a leap year, he had made an error of 23 miles in some of the numbers. This error was the cause of losing two vessels to the northward of Turk's Island, and bringing others into serious difficulties."

Bowditch, during his few years at sea, had developed a new method of computing longitude by lunar distance. The only other methods of computing longitude were from observations of the moons of Jupiter (try that in a seaway!) and by using magnetic variation charts, neither of which were accurate or easy to accomplish. Nevertheless, Bowditch discussed both methods in his book. He published his new lunar distance method in the book with this note in the preface:

"All the examples of the present work are adapted to American places. The method of constructing the problems of Middle-Latitude Sailing is more simple than in Moore's work. The Journal is entirely new. The examples of lunar observations are adapted to the year 1804, which will save the learner the expense of purchasing an old otherwise useless Nautical Almanac. A new method of working a Lunar Observation is given in this work, which was published in both American editions of Moore's treatise; it was invented by the author of this work in the year 1795, and taught by him to a number of persons in the year 1796, he not having seen any method possessing the peculiar advantage of uniformity in applying the corrections; but since that time he has seen a method somewhat similar, published by Mr. Mendozo y Rios in the Philosophical Transactions for the year 1797."

One of the "learners" Bowditch referred to as a student of his method was his ship's cook, who presumably worked out his lunar distance calculations when not in the galley. Bowditch was proud of the fact that his method was easy enough for virtually anyone to learn.

Nathaniel's accomplishments, then, were in several areas. Not only did he summarize all the important navigational knowledge of the day in one volume, he corrected his sources in the finest detail. He developed and published new methods of sight reduction. And he gave explanations and examples of each of the calculations expected of the competent seagoing navigator.

What was the effect of Bowditch's work? It can truly be said to be immeasurable, in every sense of the word. We'll never know how many American commercial and naval vessels did not go aground because they followed the improved methods and tables Bowditch published. We'll never know how many American naval ships owe their accomplishments in battle in part to the navigational skills of their illustrious captains, men like Jones and Decatur, made possible by Bowditch's patient hours of calculation and writing. Yet it is certainly true that this little known, almost obscure American changed the course of the young nation's history.

Original first editions of The New American Practical Navigator can be viewed at a number of public libraries, including the U.S. Naval Observatory in Washington, D.C., and the Mariner's Museum in Newport News, Va. Viewing one or, better yet, handling one, is an almost religious experience for anyone who appreciates American history and navigation science. There are doubtless original editions in the hands of private collectors; a recently reported price for one was $9,000. We can only hope that collectors appreciate the true value of these priceless volumes. It isn't measured in dollars.

Richard K. Hubbard works at the Defense Mapping Agency's Hydrographic/Topographic Center and was one of the editors of the recent revision of The American Practical Navigator, to be published in December as the 1995 edition.