Page 156

 Diagram:

Page 157

...he must get his longitude, also, and this, if he chooses, he can find from the same sextant sight, by means of his chronometer. 

(A chronometer is a very fine and adjusted clock. One Harrison, by inventing a clock that would keep accurate time in all temperatures, and under all circumstances, received in 1773, a reward of $100.000 offered in the reign of Queen Anne for the best method of finding a ship longitude. The method is simplicity itself, in theory, and theories are all we are after now)

You know, of course, that the earth revolves on its axis once in each 24 hours. As the earth turns from the W. to E. and the meridians of long. run N. and S., you can see that the 360° line of longitude passes under the [sun] one after the other, in 24 hours. And that means that a new degree passes every 4 minutes.

Now, it is noon at any place on the earth when the [sun] is at the highest point in the heavens at that place. That is, it is noon at any place when the meridian, which passes through that place, is directly under the [sun]. Consequently, as it takes 4 minutes for the [sun] to pass through one degree, difference of longitude becomes merely a difference of time.

Now, the captains' chronometer is kept running with the most anxious exactitude to the time of Greenwich, England, which you remember, is longitude zero. When the captain finds the noon for his locality (by noting when the sun is at its highest point, as explained, he has but to compare his time with the Greenwich time, and figure 4 minutes to each degree. So, in the case we have before us, at the moment the captain made “8 bells” – noon – someone, stationed for that purpose at the chronometer marked the exact chronometer time. Suppose that was 4:52 PM. That is 4 hours and 52 minutes after noon, or 292 minutes after noon. Counting four minutes of time to each degree of longitude, he would be 73° of longitude to the W. of Greenwich. The captain knows,..

Pagee 159

…therefore, with certainty, that his ship at noon was at that exact spot, on the chart where the 73rd meridian of W. long. crossed the 40th parallel of N. lat., because he is on both at once, and can be nowhere except at their intersection. (page 146). If his position happened to come in odd minutes instead of even degrees, he could find the place with a little measuring.

This, as you've read it, gives the theory and the actual practice in its simplest form. The real operation of "working out a sight" is not so easy, as what you've read, for there are many allowances and conditions to understand and figure out.

But with experience, skill, care and knowledge, and with a sky that will allow him frequent sights at the sun or the stars – for the stars are also used, a captain can take his steamer across the ocean as accurately as you can stear a bicycle across a park, and he can keep in the center of a narrow “steamship lane” as exactly as you can keep your wheel to the center of the gravel walk. Finis

Page 160 THE END

Diagram:

James M Miller (captain)

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