On June 6th, Darwin set foot on the Beagle again – for the first time in over a month:
“Went on board & breakfasted with the Captain, spent the day between the city & the Beagle.— Going on board gives in a small degree the comfortable feeling which is always experienced on returning home.— Having lived so long on shore, I have almost forgotten how to stow myself in my own corner.” (June 6)
I find it interesting that, even though he has dealt with terrible seasickness, he still considers the Beagle “home”, and is given comfort by returning. (Though it does sound like he has gotten a little used to spreading out his “stuff” on his own. Ah – the life of a bachelor naturalist…)
The Beagle returned to Bahai last month because FitzRoy was concerned that his longitudes were off. One of the memorandum in the orders given to him by the Lords of the Admiralty was to accurately pinpoint the longitude of Rio de Janeiro. His orders stated:
“A considerable difference still exists in the longitude of Rio de Janeiro, as determined by Captains King, Beechey, and Foster, on the one hand, and Captain W. F. Owen, Baron Roussin, and the Portuguese astronomers, on the other; and as all our meridian distances in South America are measured from thence, it becomes a matter of importance to decide between these conflicting authorities. Few vessels will have ever left this country with a better set of chronometers, both public and private, than the Beagle; and if her voyage be made in short stages, in order to detect the changes which take place in all chronometers during a continuous increase of temperature, it will probably enable us to reduce that difference within limits too small to be of much import in our future conclusions.” (as recorded in FitzRoy’s Narrative)
However, arriving in Rio and making his measurements FitzRoy found a discrepancy:
“As I found that a difference, exceeding four miles of longitude, existed between the meridian distance from Bahia to Rio, determined by the French expedition under Baron Roussin, and that measured by the Beagle; yet was unable to detect any mistake or oversight on my part; I resolved to return to Bahia, and ascertain whether the Beagle’s measurement was incorrect. Such a step was not warranted by my instructions; but I trusted to the Hydrographer for appreciating my motives, and explaining them to the Lords of the Admiralty. In a letter to Captain Beaufort, I said, “I have not the least doubt of our measurement from Bahia; but do not think that any other person would rely on this one measure only, differing widely, as it does, from that of a high authority—the Baron Roussin. By repeating it, if it should be verified, more weight will be given to other measures made by the same instruments and observers.” (FitzRoy’s Narrative)
If you recall, FitzRoy’s Chronometers where a key tool for doing accurate survey work along the coast (which is why he brought 22 of them). Finding that his measurements were off from Baron Roussin’s (see Of French Barons and Oil Seeps) raised some serious concerns. If his readings were already off after just a few month, then the rest of the data collected over the next several years would be called into question. On the other hand, if his data were more accurate than earlier explorers, then it meant that all the money he had invested in the Beagle‘s chronometers was paying off, and his maps would be the most accurate to date. So FitzRoy did what any good scientist would do – he went back to “test his hypothesis”.
This was one of the great character traits of FitzRoy – he trusted his judgement. Some captains may have pressed on, since adding a month to the trip to retake a measurement was not in their orders. But FitzRoy followed the spirit of his instructions, and often took “political risks” if it meant doing what was needed to get the job done.
So the Beagle “sailed with the ebb-tide and sea-breeze” on May 10th and headed back to get more data. Darwin gives us a timeline:
“The Beagle made a very good passage up; being only 5 days, she passed a few miles inside of the Abrolhos.— A French corvette sailed 8 days before & promised our Captain to have dinner ready for him on his arrival at Bahia; as it turned out the case was reversed; such is the advantage of a good knowledge of the winds & coast.— She staid a week at Bahia.— And 12 days back to Rio; she would have been some days shorter on the passage, had she not been becalmed at Cape Frio.” (June 4)
So what did the data have to say? I’ll let FitzRoy explain his results after return to Rio on the evening of June 3rd:
“Next day (4th) the usual sets of equal altitudes were observed; and after the chronometer rates were ascertained, I had the satisfaction of finding that this third meridian distance agreed exactly with the first and second. Upon further examination, it was seen that the Abrolhos Islands were laid down correctly in the French chart, with respect to Bahia; but that the meridian distance between those islands and Rio de Janeiro differed more than four miles from that resulting from three measures made by our twenty chronometers.” (FitzRoy’s Narrative)
So FitzRoy had been correct, and his extra trip reaffirmed that his chronometers were working well. He must have felt extremely satisfied to have his initial measurements validated.
FitzRoy wrote to the Baron to let him know of his mistake:
“A few weeks afterwards all the data and results of these measurements were given to the French Commander-in-chief on the station, who promised to forward them to the Baron Roussin; but I have heard nothing of their having been received.” (FitzRoy’s Narrative)
History records no response. (RJV)