Of his noctograph Prescott made constant use. For composition he employed it almost altogether, seldom or never dictating to a scribe. Obviously, however, the instrument allowed no erasures or corrections to be made, and the writer must go straight forward with his task; since to go back and try to alter what had been once set down would make the whole illegible. Hence arose the necessity of what Irving once described as "pre-thinking,"--the determination not only of the content but of the actual form of the sentence before it should be written down. In this pre-thinking Prescott showed a power of memory and of visualisation that was really wonderful. To carry in his mind the whole of what had been read over to him in a session of several hours,--names, dates, facts, authorities,--and then to shape his narrative, sentence by sentence, before setting down a word, and, finally, to bear in mind the whole structure of each succeeding paragraph and the form in which they had been carefully built up--this was, indeed, an intellectual and literary achievement of an unusual character. Of course, such a power as this did not come of itself, but was slowly gained by persistent practice and unwearied effort. His personal memoranda show this: "Think closely," he writes, "gradually concentrating the circle of thought." And again: "Think continuously and closely before taking up my pen. Make corrections chiefly in my own mind." And still again: "Never take up my pen until I have travelled over the subject so often that I can write almost from memory." But in 1827, the time had not yet come for composition. He was hearing books read to him and was taking copious notes. How copious these were, his different secretaries have told; and besides, great masses of them have been preserved as testimony to the minute and patient labour of the man who made and used them. As his reader went on, Prescott would say, "Mark that!" whenever anything seemed to him especially significant. These marked passages were later copied out in a large clear hand for future reference. When the time came, they would be read, studied, compared, verified, and digested. Sometimes he spent as much as five days in thus mastering the notes collected for a single chapter. Then at least another day would be given to reflection and (probably) to composition, while from five to nine days more might go to the actual writing out of the text. This power of Prescott's increased with constant exercise. Later, he was able to carry in his head the whole of the first and second chapters of his _Conquest of Peru_ (nearly sixty pages) before committing them to paper, and in preparing his last work, _Philip II._, he composed and memorised the whole fifth, sixth, and seventh chapters of Book II., amounting to seventy-two printed pages.