It’s not even a page in length; there is just one paragraph that makes clear Hutton is inventing a new type of line: “This method was the connecting together by a faint line all the points which were of the same relative altitude… the relative altitude of all the parts being known; and as every base or little place had several of them passing through it, I was thereby able to determine the altitude belonging to each space with much ease and accuracy.”
I imagined him pencil and ruler to hand slowly joining the dots that made up his spot height bases, and that with the help of Prof. Robin Johnson I could now follow his method and replicate the missing map. I prepared my stout piece of paper: “four feet long by four feet broad” placed upon it all the points for which heights had been calculated, reversing two number sequences that ‘read the wrong way’: I doubt it was Robin’s mistake, with so many hands in play – the surveyors, the astronomer royal and Hutton himself – it would be easy for a set to be copied in the wrong order from one notebook to another.
It seemed easy enough to mark lines at 100ft elevation, and in plenty of places those for 50ft but I think either he didn’t publish everything or it was really a boast too far when in the next sentence he states: “In this estimation I could generally be pretty sure of the altitude to within ten feet and often within five feet”.Even with the multitude of spot heights at my disposal interpolation was not straight forward, there were a number of places where the ‘either/or’ of bifurcation tempted, though I chose not to (come to the Lit and Phil and I’ll explain). But am not so sure Hutton wouldn’t have employed the occasional branching line. It’s strange how full it looks, but even OS maps 70 years later had much less data to play with. Hutton’s surveyors provided a wealth of material to work from (unlike the French ha ha! I will come onto them… eventually).
With this map near complete, I have begun work on ‘compare and contrast’ models: Schiehallion in contours á la Hutton versus modern OS. But now, today, weather permitting (this time), it’s back to the mountain, and with the aid of the John Muir Trust, a mission to find the sites of the astronomer royal’s observatories.