From this great variety, which the surfaces of lakes assume, we may draw this conclusion, that the painter may take great liberties, in point of light and shade, in his representation of water. It is, in many cases, under no rule, that we are acquainted with; or under rules so lax, that the imagination is left very much at large.vol. i, p. 100
Who else could it be but that arbiter of the eighteenth-century picturesque taste, the Revd William Gilpin? Many guide-book writers had preceded him—Johnson, Boswell, and Thomas Gray among them—but only he, as early as 1772, could have written in terms of granting the artist the license to 'take great liberties' with what Nature put before him. It is a aesthetic leap that marks him out as a great deal more than just another dilettante; he was only one step away from the full-bloodedly romantic conception of a creative imagination that half-creates and half-perceives. There were times when he could be amusingly literal about the artist's right to alter Nature to the requirements of the picturesque—as when, in his guide book on the Wye valley, he incites the enthusiastic tourist to correct the unsightly gable ends in the ruins of Tintern Abbey:
A mallet judiciously used (but who durst use it?) might be of service in fracturing some of them; particularly those of the cross isles, which are not only disagreeable in themselves, but confound the perspective. 
Wordsworth read the Observations on Cumberland and Westmoreland as a teenager at Hawkshead when it first appeared. It had an immediate and formative impact on his big poem of the moment, The Vale of Esthwaite , and would influence the way he conceived of imaginative thought for the rest of his life. When, in the 1815 Preface, he described "the conferring, the abstracting, and the modifying powers of the imagination," he wrote as the inheritor of a intellectual tradition that begins with Gilpin. Wordsworth wasn't the only writer whose thought was shaped by Gilpin, but he's the most important. The young Samuel Rogers, touring the Lakes in 1789, took with him the Observations so as to inform his search for 'picturesque beauty', and Gilpin is invoked on numerous occasions in Austen, often for comic effect, as when Elinor guys Marianne in Sense and Sensibility about the way in which one meeting more will suffice to explain Mr Willoughby's opinions on the picturesque. By summer 1800, when Coleridge recorded in a notebook his disgust at 'Ladies reading Gilpins etc., while passing by the very places instead of looking at the places',  his ideas had permeated the culture. Acolytes like Uvedale Price were already appropriating them in more complex intellectual configurations; as Gilpin confided to a friend who had asked him about Price's redefinition of the picturesque:
. . . to tell you the truth, I am so little interested about the matter, that I do not so much as know what his definition is, having never seen his book. If he explains the picturesque better than I do, he is welcome; and if I should see his definition, and think it wrong, I will never battle with him. 
He was a prolific writer, and published a number of books of considerable significance, but the Observations on Cumberland and Westmoreland is his most important in terms of its impact on the mainstream of romantic thought. It was through his eyes that visitors to the area—and, in fact, natives such as Wordsworth (Gilpin himself was a Cumbrian, of course) apprehended the shapes and forms of the landscape. More importantly, it licensed and legitimised that shaping spirit which the romantics, as contrasted with their eighteenth century forbears, came to value so highly.
Reading the Observations now, it's hard not to be struck by how prescriptive he is—in a manner that seems quite untypical of the Romantics. What can Wordsworth have thought of Gilpin's categorizing of mountains in terms of their shape ('round swelling line, without any break', 'easy line', 'lumpish forms', 'parallel lines')?  And how pedantic he sometimes appears, in a distinctly eighteenth-century manner—as when he defines fens, as distinguished from pools and lakes. But is there more than a touch of self-irony in his remark that "The fen is a plashy inundation"?  He was too much a poet to surrender to theory for long—hence, no doubt, his lack of interest in Price. His prose descriptions of the landscape are interlarded with quotations from Virgil, Ovid, Thomson, John Dyer, and many others; sometimes, as in his comments on the surfaces of lakes, he will simply give up and turn to the superior skills of the poet: "In this observation I do little more than translate from Ovid." 
Which is not to say that, at his best, he's anything less than an astonishing prose writer. The Observations on Cumberland and Westmoreland contains a full complement of inspiration. This digression on the nature of imaginative power takes a sentence to heat up, and then takes flight:
We may remark further, that the power which the imagination hath over these scenes, is not greater, than the power, which they have over the imagination. No tame country, however beautiful, however adorned, can distend the mind, like this awful, and majestic scenery. The wild sallies of untutored genius often strike the imagination more, than the most correct effusions of cultivated parts. Tho the eye therefore might take more pleasure in a view (considered merely in a picturesque light) when a little adorned by the hand of art; yet I much doubt, whether such a view would have that strong effect on the imagination; as when rough with all its bold irregularities about it; when beauty, and deformity, grandeur and horror, mingled together. strike the mind with a thousand opposing ideas; and like chymical infusions of an opposite nature, produce an effervescence, which no harmonious mixtures could produce.
Surely there is a hidden power, that reigns
'Mid the lone majesty of untamed nature,
Controuling sober reason————- 
It would be hard to find a passage more attractive to the popular imagination in the final decades of the eighteenth century. It was with good reason, when she wrote her Gothic chiller Ethelinde , that Charlotte Smith set part of it in Grasmere. Or that Ann Radcliffe, when she wrote her own travel book in 1794, ensured that it covered the Lakes. It was to experience for themselves this intoxicating cocktail of "beauty, and deformity, grandeur and horror", that swarms of tourists flocked there in the years following publication of the Observations . They too wanted to engage with that "hidden power" that, to the tourist's delight, inhered within a natural world that was felt to be dangerous, horrific, and, in some obscure but thrilling sense, beyond human control. Now that Cumbria is the home of nuclear power and has been made accessible by a network of trunk roads, that's a sensation far less familiar to us—except, perhaps, when people suffer hypothermia, or in some cases, lose their lives, on the fells.
If this handsome volume is anything to go by, the science of facsimile editions has no farther to go. The typeface is as sharp as can be, and the illustrations are beautifully reproduced, in full colour. The text is introduced by Jonathan Wordsworth—shrewd and persuasive, as always. For anyone without access to an actual copy of the first edition—and even for those who do have one, but wish to preserve it—this is an essential acquisition, its price amply justified both by its significance, and the care with which it is printed.
Observations on the River Wye (1782) p. 33.
Samuel Taylor Coleridge, Notebooks , ed. Kathleen Coburn (5 vols., Princeton. 1957- ) vol. I, p. 260.
Carl Paul Barbier, Samuel Rogers and William Gilpin: Their Friendship and Correspondence (Oxford, 1959) pp. 1-2.
Observations vol. i, p. 83.
Observations vol. i, p. 93.
Observations vol. i, p. 99.
Observations vol. i, pp. 121-2.