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Bha mi ag innse dhuibh mun deuchainn shaidheansail air Sìth Chailleann ann an seachd ceud deug, seachdad ʼs a ceithir (1774). Bha an Reultair Rìoghail, Nevil Maskelyne, a tomhas cuideam na cruinne.

Bha an samhradh sin air leth fliuch agus fuar. Ach, gu fortanach, bha oidhcheannan ann nuair bha na rionnagan a-muigh. Rinn Maskelyne tomhas orra leis an uidheamachd a b’ fheàrr san t-saoghal.

ÌýÌýÌýÌýÌýÌýÌýÌýÌýÌýÌý Rinn e tomhas air a’ cheàrn no angle gu rionnagan bhon ionad-sheallaidh aige air cliathaich a deas na beinne. An uair sin, rinn e an aon rud às ionad eile air cliathaich a tuath na beinne. Bha an dà ionad faisg air mìle air falbh o chèile. Bha fios aig Maskelyne dè an diofar a bha ann an domhan-leud – no latitude – eadar an dà àite. Le sin, dh’obraich e a-mach dè an diofar a bu chòir a bhith anns a’ cheàrn do na h-aon rionnagan eadar gach ionad-seallaidh.

ÌýÌýÌýÌýÌýÌýÌýÌýÌýÌýÌý Ach cha robh am figear a fhuair e co-ionann ri sin. Carson? Uill, bha e a’ tomhas dìreachdas le crochadan, no pendulum. Mar a bha e an dùil, bha a’ bheinn a’ tarraing air a’ chrochadan pìos beag. Agus bhon diofar eadar an dà àireamh, dh’obraich Maskelyne a-mach dè cho trom ʼs a bha Sìth Chailleann. An uair sin, fhuair e figear airson cuideam na cruinne gu lèir. Agus cha robh e fada ceàrr a rèir mar a tha daoine a’ tomhas cuideam na cruinne an-diugh.

ÌýÌýÌýÌýÌýÌýÌýÌýÌýÌýÌý Fhuair Maskelyne na Gàidheil gu math sìobhalta agus carthannach. Sgrìobh e ‘their country is with justice celebrated for its hospitality and attention to strangers’. Am measg nan daoine air an d’ fhuair e eòlas, bha fear Donnchadh MacDhonnchaidh. Bha e mar sgalag tron latha, a’ giùlan eallachan suas is sìos a’ bheinn. Ach bha e cuideachd na fhìdhlear air leth. Air an fheasgar bha e a’ cluich rìdhlichean is sigichean don a h-uile duine.

I was telling you about the scientific experiment on Schiehallion in 1774. The Astronomer Royal, Nevil Maskelyne, was measuring the mass of the Earth.

ÌýÌýÌýÌýÌýÌýÌý That summer was particularly wet and cold. But, fortunately, there were nights when the stars were out. Maskelyne measured them with the best equipment in the world.

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ÌýÌýÌýÌýÌýÌýÌý He measured the angle to stars from his observatory on the southern slope of the mountain. Then he did the same thing from another observatory on the northern slope of the mountain. The two places were nearly aÌý mile from each other. Maskelyne knew what the difference in latitude was between the two places. With that, he worked out what difference there should be in the angle to the same stars between each observatory.

ÌýÌýÌýÌýÌýÌýÌý But the figure he achieved wasn’t equivalent to that. Why? Well, he was measuring verticality with a pendulum. As he [had] expected, the mountain was attracting the pendulum a small amount. And, from the difference between the two numbers, Maskelyne worked out how heavy Schiehallion was. Then he obtained a figure for the mass of the entire Earth. And he wasn’t far wrong, according to how people measure the mass of the Earth today.

ÌýÌýÌýÌýÌýÌýÌý Maskelyne found the Gaels very civilized and charitable. He wrote ‘their country is with justice celebrated for its hospitality and attention to strangers’. Among the people he got to know was Duncan Robertson. He was a servant through the day, carrying loads up and down the mountain. But he was also an excellent fiddler. In the evening, he was playing reels and jigs for everyone.