It sometimes happens that I overlook the introduction of a person to my wider acquaintance – not from any desire to keep that person to myself, but simply because he, or she, plays such a central role in my life that I have assumed that my acquaintances were all familiar with her, or him.

So it is with me and Mr L of Toronto, a man so devoted to his planting that he comes coiffed as a shrub. Just as the Duchess of Devonshire wore a kitchen garden on her head on special occasions, so has Mr L polished the crown of his head to a smoothness that a groundsman would envy, and planted it about with a thick shrubbery of hair that is ornamented only by his two perfectly formed ears – a man in short whose knowledge and commitment are as a deep river and its bed to the purling rill of the Brown Advisor.

Nonetheless he has condescended to a question: what do I know about the surveying techniques of that glorious place-maker, Capability Brown?

The answer is on the one hand, very little, and on the other, a contention that without the theodolite it would not have been possible for Brown to design as he did.

Let us begin with the evidence: Brown’s successor, Humphry Repton, claimed to handle a theodolite and showed himself at it on his trade-plate. However I have little by way of evidence of his skill and do know that he relied on existing plans wherever he could get them. As for Brown, he did not rely exclusively on Lapidge and Spyers, but where there were existing plans he too would use them and – apparently – sketch on top of them.

However skill with a theodolite appears to have been an essential. I fancy that Brown’s predecessor, Charles Bridgeman might have left one in his will, but Captain Ken reminds me that surveying was a polite accompaniment, and Mr H adds that surveying was very much au courant for ladies too. But I have to hand a characteristic letter on the subject:

‘To Mr. Thomas Duffield at Wallington

Sir Walter Blackett bids me write you that Lady Loraine is so very pressing for the loan of his Theodolite again he could wish Mr. Forster and you could contrive to finish the survey of the most material part of Cock’s farm against Saturday next and leave the rest till the Spring and so let the Lady have the instrument for the present.’

Thomas Wright, the Wizard of Durham, the first man to realise that the Milky Way was a belt of stars and that our solar system lay within it, travelled from house to house (including Wallington) under his own ragged clouds of depression and madness dispensing lessons in mathematics and surveying, as he designed gardens and buildings. Thus one finds in his diary a note on making a survey of the garden at Wrest alongside mention of his teaching Lady Grey and her children. Surveying was taught at Eton to generals and admirals in the making, who would require the rudiments for building fortifications and navigation, and whatever the reputed limits of their interest, the rural gentry of the 18th century did look beyond killing and planting, and Sir Walter Blackett of Wallington was not the only one to take an interest. William Windham III of Felbrigg, the statesman, friend of Burke, and patron of Repton, was one of those politicians who might have been great but was like Wright undone by periods of depression and indecision and by his lifelong passion for the wife of John Byng, Viscount Torrington, that old friend of both Brown and Repton: ‘Of all his books in the library at Felbrigg, the treatises on mathematics ancient and modern are the most heavily annotated’ – a resort one hopes for a heavy and broken heart.

Here for a third example is Peter Collinson’s obituary letter (3 July 1742) for the great Robert Edward, 9th Lord Petre, praised not only as a gardener but as ‘a great mechanic, as well as a great mathematician; ready at figures and calculations – and elegant in his tastes’.

In short the theodolite provided accurate ways of measuring angles and easy ways of setting them out on the ground, which in turn could bring a degree of symmetry to a landscape and a happy way of distributing clumps.