angular distance between the centres of the moon and of the sun, or of the moon and some star.
The motion of the moon is so rapid that this angular distance changes from second to second, and thereby, by previous astronomical calculation, the time at Greenwich at which its distance from any body is a certain number of degrees can be ascertained and recorded.
By well-known calculations the local time at any spot can be obtained, and when this is ascertained, at the precise moment that the angular distance of sun and moon is observed, the difference gives the longitude.
This seems simple enough, but there is a good deal of calculation to go through before the result is reached, and neither the observation nor the calculation is easy, especially with the astronomical tables of those days, and there were very few sailors who were capable of, or patient enough to make them, nor was the result, as a rule, very accurate. For one thing, the motions of the moon, which are extremely complicated, were not enough known to allow her calculated position in the heavens to be very accurate, and a very small error in this position considerably affects the time, and therefore the longitude.
Luckily for Cook, the Nautical Almanac had just been started, and contained tables of the moon which had not previously been available, and which much lightened the calculations.
The great invention of the chronometer, that is, a watch that can be trusted to keep a steady rate for long periods, was at this time completed by Harrison; but very few had been manufactured, and astronomers and sailors were slow to believe in the efficacy of this method of carrying time about with a ship. Thus Cook had no chronometer supplied to him.
Green had accompanied Mr. Maskelyne, afterwards Astronomer Royal, to Barbados in 1763 in H.M.S. Princess Louisa, in order to test Harrison's timekeeper, and also a complicated chair, from which it was supposed observations of Jupiter's satellites could be observed on board ship; and as this trial afforded the final triumph of the new method, one would have thought that on a voyage of circumnavigation he would have made every effort to get one of these watches.
Be this as it may, the Endeavour had no chronometer, and lunars were the mainstay of the expedition.
In these observations Green was indefatigable. Cook, an excellent observer himself frequently took part in them; but it was Green's especial business, and no doubt to him is due the major part of the determinations of accurate longitude, which is one of the very remarkable points of this voyage.
Green's log, which is extant, is filled with lunar observations, and the extraordinary coincidence between different observations attests the care with which they were made. I dwell upon this because, while full of admiration for Cook's knowledge, and his untiring zeal in every detail of his expedition, it is evident, from a study of the original documents, that without Green many opportunities of getting longitude would have been lost, Cook having no time to spare to make use of them. Let us give honour to whom honour is due.
The final results of the observations are not equally good, but this arises from the errors, before referred to, in the moon's place in the heavens as given in the almanac, which would vary with her position, and affect the longitude accordingly. The astonishing thing is, not that some longitudes are considerably in error, but that the majority of them are so near the truth.
The Endeavour sailed from the Thames on June 30th, 1768, and was in Plymouth Sound from July 14th to the 26th, when she finally sailed, Banks and the scientific staff having joined here.
She carried a complement, all told, of ninety-four, and very close stowage it must have been.
A list is given in this book, immediately before the "Journal," of every person on board when the ship sailed from Plymouth.
The draught of the ship was 13 feet 6 inches, and her provisions were calculated to last eighteen months. The original intention had been that the transit of Venus should be observed at the Marquesas; but the Dolphin's return before Cook sailed, with the news of the discovery of Tahiti and its friendly inhabitants, caused this island to be finally selected.
The exact text of Cook's orders cannot be given. They were secret orders; but, curiously enough, while the covering letter, which enjoined him to show them to nobody, which is dated July 30th, 1768, is duly entered in Admiralty Records, the orders themselves, which should follow in the letter book, are omitted. They have never been published. Nevertheless, we can gather what they were.
Cook, in the published account of his Second Voyage, says he had instructions to proceed directly to Tahiti, and afterwards to prosecute the design of making discoveries in the Pacific by proceeding southward to the latitude of 40 degrees, and if he did not find land to continue his voyage to the west till he fell in with New Zealand, which he was directed to explore, and thence to return to England by such route as he should judge most convenient.
Precautions against the terrible scourge, scurvy, had not been forgotten.
Besides the supply of all anti-scorbutics then known, a special letter was written to Cook directing him to take a quantity of malt to sea, for the purpose of being made into wort, as a cure for scorbutic disorders, as recommended by Dr. McBride.
The directions for its use were as follows:--
"The malt must be ground under the direction of the surgeon, and made into wort, fresh every day, in the following manner:--
"1. Take one quart of ground malt, and pour on it three quarts of boiling water. Stir them well, and let the mixture stand close covered up for three or four hours, after which strain off the liquor.
"2. The wort, so prepared, is then to be boiled into a panada, with sea biscuit or dried fruits generally carried to sea.
"3. The patient must make at least two meals a day of the said panada, and should drink a quart or more of the fresh infusion as it may agree with him, every twenty-four hours.
"4. The surgeon is to keep an exact account of its effects."
Though it is somewhat anticipating events, it is convenient to record here the result of these efforts to defeat the hitherto unconquerable enemy. Mr. Perry's report at the termination of the voyage is as follows:--
"Sour krout, mustard, vinegar, wheat, inspissated orange and lemon juices, saloup, portable soup, sugar, molasses, vegetables (at all times when they could be got) were, some in constant, others in occasional use. These were of such infinite service to the people in preserving them from a scorbutic taint, that the use of the malt was (with respect to necessity) almost entirely precluded.
"Again cold bathing was encouraged and enforced by example; the allowance of salt beef and pork was abridged from nearly the beginning of the voyage, and the sailors' usual custom of mixing the salt beef fat with their flour, etc., was strictly forbad.
"Upon our leaving England, also, a stop was put to our issuing butter and cheese, and throughout the voyage raisins were served with the flour instead of pickled suet. At Tierra del Fuego we collected wild celery, and every morning our breakfast was made with this herb, with ground wheat and portable soup.
"We passed Cape Horn, all our men as free from scurvy as on our sailing from Plymouth.
"Three slight cases of scorbutic disorders occurred before arriving at Otaheite. Wort was given, with apparently good effect, and the symptoms disappeared.
"No other cases occurred during the voyage, but the wort was served out at sea as a regular article of diet."
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