1 in 500k map showing Perth, Aberfeldy and Loch Rannoch |
1 in 100k map showing Schiehallion and Loch Rannoch |
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Maskelyne on Schiehallion
or one man' s geophysical year. Summary
As the International Geophysical Year gets under way we have heard enough from the Antarctic, from the weather ships, and other places to know that the scientists involved are prepared for a fair amount of plain physical discomfort before their work is finished.
Perhaps it has always been so. Certainly when the Reverend Nevil Maskelyne, George III's Astronomer Royal, crossed General Wade’s bridge at Aberfeldy on the 30th of June in 1774 to spend 161/2 weeks living high up on the sides of the mountain Schiehallion, he had no doubt that the scientific observations he was to carry on "in so inclement and inconvenient a situation" would be attended by difficulties and fatigues.
In those days, of course, the Highlands of Scotland were some days' journey from London - Maskelyne's apparatus had to come by sea as far as Perth, and completed its journey up the side of the 3400 foot mountain on the shoulders of strong - and probably rather puzzled - men. Maskelyne himself at the age of 42, was a man reputedly of mild and genial temper, fond of good living when opportunity offered, but quite prepared to suffer in the cause of science. He had previously journeyed to St. Helena to make astronomical observations there, only to be frustrated by bad weather. A few years later he had sailed to the Barbados and back to test chronometers. So his journey to the Highlands probably seemed less forbidding to him than it might have done to many others; and in the event he seems to have had a reasonably sociable time. His arrival in Scotland was reported by the Scots Magazine, and he was able subsequently to acknowledge with pleasure - "the civilities of all the neighbouring gentlemen, who often paid me visits on the hill, and gave me the fullest conviction that their country is with justice celebrated for its hospitality and attention to strangers."The aim of the experiment Maskelyne set out to do is now usually said to be weighing the earth though Maskelyne called it rather delightfully Measuring the attraction of some hill. The idea wasn't new; it had been tried unsuccessfully by a French expedition in the Andes in 1738, and seems to have been suggested first by Sir Isaac Newton. Maskelyne's experiment on Schiehallion was the first successful one, and set the pattern for later repetitions of the experiment elsewhere -- for instance, on Arthur's Seat in Edinburgh. The idea of the experiment is very simple. If you hang a weight from a string, we say that the string hangs vertically, and builders, paper-hangers, surveyors and anyone else who wants to know the true vertical find it in this way, using a plumb-line. But suppose, that when a plumb-line is hanging vertically, a very heavy weight is brought up to one side of the plumb-bob; then as well as being pulled in a down-ward direction by the mass of the earth beneath it, the bob will be pulled sideways by the heavy weight beside it, and so it hangs slightly off the true vertical line. If you work it out you find that if a mountain the size of Schiehallion has the same average density as the rest of the earth its effect on a plumb-line hanging just beside it is to pull the plumb-line 1/360th. of a degree away from the vertical. If the plumb-line is six feet long that means the bob is pulled sideways about 4 thousandths of an inch. That’s not very much; and of course it's difficult to detect - you can’t take the mountain away and see whether the bob moves, and you can't find how much the Plumb-line is out of truth by hanging another one up beside it, because it’ll be affected the same way. But you can find the true vertical by observing the directions of the fixed stars, and this of course is how it became a job for the Astronomer Royal. What Maskelyne did was roughly this: He set up two observing stations or observatories, one on the steep southern slope, the other on the northern slope, of the wedge-shaped mountain, and from each observatory he measured the angle between the direction of a plumb-line and the positions of a number of stars at their maximum altitudes in the night sky. There was a systematic difference between the readings taken at the two observatories. Part of this difference was due to the fact that one observatory was about ¾ mile north of the other, and so at a different geographical latitude. The rest of the discrepancy was due to the deflection of the plumb-line by the attraction of the mountain. Neither plumb-line was truly vertical one being pulled northwards and the other southwards and Maskelyne found the sum of these deflections. His narrative of the experiment from its origins to its successful conclusion was published in Vol. 65 of a journal whose modern title is The philosophical transactions of the Royal Society; in those more leisured days the title-page bore the legend Philosophical transactions, giving some account of the present undertakings, studies and labours of the ingenious in many parts of the world. It was in 1772 that Maskelyne proposed this experiment, but at that time he had little idea where it should be carried out. Accordingly, he reports - Mr. Charles Mason, who had been employed on several astronomical occasions by the Royal Society, was appointed to make a tour of the Highlands of Scotland in the summer of 1773, taking notice of the principal hills in England which lay in his route, either in his going or in his return. It appeared from his observations that scarce any hill was so well adapted to the purpose as our more sanguine hopes had led us to expect; for either they were not high enough, or not sufficiently detached from other hills, or their greatest length fell in a wrong direction, instead of lying nearly east and west, which is a circumstance requisite to make a hill of a given height afford the greatest effect of attraction. Fortunately, however Perthshire afforded us a remarkable hill, nearly in the centre of Scotland, of sufficient height, tolerably detached from other hills, and considerably larger from east to west than from north to south, called by the people of the low country Maiden-Pap, but by the neighbouring inhabitants Schiehallion, which I have since been informed signifies in the Erse language Constant Storm; a name well adapted to the appearance which it so frequently exhibits to those who live near it, by the clouds and mists which usually crown its summit.So Schiehallion was chosen - with all its clouds and mists and now funds had to be obtained. "It was foreseen," he says, "that this experiment would be attended with considerable expense, and such as might easily have exceeded the common funds of the Royal Society without some extraordinary assistance. The bounty of His Majesty our patron happily removed this difficulty." Apparently George III had put up some money for astronomical research in 1769, and not all of it had been spent. The Society thought they could not dispose of it on any more important objects or in any manner more consistent with the intentions of their Royal Patron and Benefactor than by expending it on this astronomical experiment of the attraction of a mountain, as what ..... was likely to lead to new discoveries concerning the constitution of this earth which we inhabit, particularly with respect to the density of its internal parts.The next problem was to decide who should be in charge of these "very important observations". With a very becoming modesty, Maskelyne reports that "numerous and interesting as my literary engagements are at the Royal Observatory, I had no thoughts of undertaking this care and labour myself, till the Council of the Royal Society were pleased to do me the honour to think my assistance necessary to insure the success of so important and delicate an experiment. Their thinking so was a sufficient motive with me to encounter whatever difficulties and fatigues might attend operations carried on in so inconvenient and inclement a situation."Maskelyne arrived at Schiehallion on the 30th of June, 1774, and found his southern observatory ready for him. "The observatory was fixed half-way up the side of the hill, as the place where the effect of the hill's attraction would be at the greatest" - this by the way was a piece of guesswork; a few years later it was shown. mathematically that a larger effect would have been obtained if the stations had been lower down, in which case the "difficulties and fatigues" might have been a bit less, too. "A circular wall was raised, five feet in diameter, and covered at top with a movable conical roof for sheltering the astronomical quadrant, and a square tent was put up for receiving the transit instrument. A bothie, or temporary hut, was also made near it, for my residence, while I was attending the astronomical observations on this side of the hill."The observatory was in fact at an altitude of about 2300 feet, and Maskelyne lived there for seven weeks. "Through the badness of the weather," he reports, "which was almost continuously cloudy or misty," the first three weeks yielded little in the way of results; but while Maskelyne was more or less idle on the mountain a team of surveyors was out - weather permitting - doing a detailed and accurate survey of the hill and the area round it, so that the positions of the observatories would be known, and so that the size and volume of the mountain could be calculated. By mid-August Maskelyne was satisfied with the observations he had made at the southern observatory and was ready for his instruments to be moved to the northern station. "This was a work of great labour and difficulty as everything was carried over the ridge of the hill on men's shoulders and some of the packages were very weighty; it employed the labour of twelve men for a week. A large level area had been cut away here in the side of the hills for receiving the observatory. A new bothy was also erected, and places for holding the quadrant, and transit instrument, as before, adjoining to the observatory."This site can still be identified on the hillside and shows unmistakable traces of Maskelyne's occupation. Two streams which cut straight down the hill-side form an unmistakable feature of the northern aspect of the hill as seen from General Wade’s road. At about 2300 feet, and roughly 150 yards west of these streams, there are two rectangular level platforms, 5 or 6 yards wide and 10 to 15 yards long. At the ends of one are the remains of the stone end walls of the bothy - the side walls were probably earth or turf. On the other platform you can still trace as a different texture of the turf, a five-foot diameter circle where the circular wall to house the astronomical quadrant was raised. Here again Maskelyne had to kick his heels for over a week before he had a clear night, and it was over three weeks before he had any sight of the sun. Indeed he remarks that "the opportunities of weather fit for observing, at all were but very rare". By the 24th of October, though, 161/2 weeks after his arrival, the astronomical measurements were complete. The survey of the mountain had been going on in the meantime, as quickly as the weather would allow - "all the people of the country agreed that it was the worst season that had ever been known," he said. But isn't the visitor always told that! Baselines had been measured out, one in Glenmore, to the south, and the other on the low ground to the north where Dunalasdair Reservoir now is. This was such an important part of the work that Maskelyne supervised it himself. The triangulations by which the size of the mountain was found in terms of the length of the baseline were done in very great detail, and with breaks during the winter months weren't finished until 1776, two vears later. The detailed and skilful labour of computation, to translate the theodolite readings into a map of Schiehallion -- this was probably the first contour map ever drawn, by the way -- was performed by Dr. Charles Hutton, F.R.S. of the Royal Military College. Maskelyne's account of his astronomical observations and Hutton's description of the surveys and of the calculations which followed it, are impressive in their care and attention to detail. Such labours deserve the reward of a successful outcome, and by July 1775 Maskelyne was able to report to the Royal Society that the experiment had been successful and showed that - "the mean density of the earth is at least double of that at the surface, and consequently that the density of the internal parts is much greater than near the surface."This, he points out, "is totally contrary to the hypothesis of some naturalists who suppose the earth to be only a great hollow shell of matter; supporting itself from the property of an arch, with an immense vacuity in the midst of it."Concluding his report, Maskelyne points out that repetitions of the experiments in different circumstances are needed to confirm his result, "but whatever experiments of this kind be made hereafter let it be always gratefully remembered that the world is indebted for the first satisfactory one to the learned zeal of the Royal Society, supported by the munificence of George the Third."Though dwarfed by the resources now marshalled for the International Geophysical Year, by the standards of the time this had been a prodigious effort, and it was the first successful step in the continuing quest for information about the internal constitution of the earth. --------------------- © Richard M Sillitto, 1957 In 1999 ownership of the eastern part of the mountain passed to the John Muir Trust. You can find out a lot more about it on the Trust's website, and buy superb postcards and pictures from their on-line shop. |