Although the labours of Kepler were frequently interrupted by severe and long-continued indisposition, as well as by the pecuniary embarrassments in which he was constantly involved, yet the ardour and power of his mind enabled him to surmount all the difficulties of his position. Not only did he bring to a successful completion the leading inquiries which he had begun, but he found leisure for composing an immense number of works more or less connected with the subject of his studies. Between 1594, when he published his Kalendar at Gratz, and 1630, the year of his death, he published no fewer than _thirty-three_ separate works; and he left behind him _twenty-two_ volumes of manuscripts, _seven_ of which contain his epistolary correspondence. The celebrated astronomer Hevelius, who was a cotemporary of Louis Kepler, purchased all these manuscripts from Kepler's representatives. At the death of Hevelius they were bought by M. Gottlieb Hansch, a zealous mathematician, who was desirous of giving them to the world. For this purpose he issued a prospectus in 1714 for publishing them by subscription, in 22 volumes folio; but this plan having failed, he was introduced to Charles VI., who liberally obtained for him 1000 ducats to defray the expense of the publication, and an annual pension of 300 florins. With such encouragement, Hansch published in 1718, in one volume folio, the correspondence of Kepler, entitled "_Epistolae ad Joannem Keplerum, insertis ad easdem responsionibus Keplerianis, quidquid hactenus reperiri potuerunt, opus novum, et cum Jo. Kepleri vita._" The expenses of this volume unfortunately exhausted the 1000 ducats which had been granted by the Emperor, and, instead of being able to publish the rest of the MSS., Hansch was under the necessity of pledging them for 828 florins. Under these difficulties he addressed himself in vain to the celebrated Wolfius, to the Royal Society of London, and to other bodies that were likely to interest themselves in such a subject. In 1761, when M. De Murr of Nuremberg was in London, he made great exertions to obtain the MSS., and Dr Bradley is said to have been on the eve of purchasing them. The competition probably raised the demands of the proprietor, in whose hands they continued for many years. In 1773 they were offered for 4000 francs, and sometime afterwards M. De Murr purchased them for the Imperial Academy of Sciences at St Petersburg, in whose library they still remain. Euler, Lexell, and Kraft undertook the task of examining them, and selecting those that were best fitted for publication, but we believe that no steps have yet been taken for executing this task, nor are we aware that science would derive any advantage from its completion.