3. He made additions to the doctrine of pendu lums, and he showed that a natural standard for measure may be obtained from the common pendu lum.
4. TIe made many improvements in the con struction and use of astronomical instruments. Ile fitted and suspended quadrants, sextants, Sze., in an improved manner. Ile made two telescopes open on a joint like a sector, to take distances to half minutes, Er.c. He made improvements on micro meter screws and reticula, for taking small dis tances, and apparent diameters of the planets, ke. to seconds, and he made variable repertures to fit glasses for crepusculine observations. He made maps of the Pleiades, and proposed to prove the earth's motion by examining the circumpolar stars.
5. In optics he made many improvements. He attempted, with some success, to make glasses of other figures than spherical. He measured and delineated the spheres of the humours of the eye. He explained the reasons of erect vision. He gave a new theory of fractions which agreed with obser vation; and he demonstrated the general phenomena of dioptrics, and the principles of refracting tele scopes.
6. He made many useful suggestions and experi ments on navigation. He made a magnetic terella, which he placed in the middle of a plain board, with a hole into which the terella is half sunk, like a globe with its poles in the horizon; steel-filings were then scattered with a sieve over the plane board, and they immediately arranged themselves into a sort of helix, proceeding, as it were, out of one pole, and returning in lsy the other.
In the practice of seamanship lie made many ex periments. He investigated the theory of sailing in all winds,—the geometrical mechanism of row ing, swimming, and flying,—and the resistance which bodies experienced in moving through fluid media.
7. In the useful arts his contrivances were nu merous.. He invented the art of mezzotinto engra ving, an art which Prince Rupert greatly improved. He suggested many useful improvements in water works. He invented the art of drawing pictures by microscopical glasses. He constructed per petual, or long-lived lamps, for maintaining a constant heat, for the purpose of hatching insects, rearing plants, preserving chemical preparations, imitating nature in producing fossils and minerals, and keeping the motion of watches equal for finding the longitude, and for other astronomical purposes.
In 1669 he communicated to the Royal Society a description of an instrument for drawing the outlines of any object in perspective. It is published in the Transactions, No. 45, p. 898.
In the same year he communicated to the So ciety, a paper entitled the Generation of an Hyper bolical Cylindroid, and a hint of its application for grinding hyperbolic glasses.—See Philosophical Transactions, No. 48, p. 961.
Sir Christopher afterwards resumed the conside ration of the subject, and communicated the draught of an engine for grinding hyperbolical glasses. See Philosophical Transactions, No. 53, p. 1059.
The idea contained in these two papers is inge nious and beautiful, but we fear not likely to answer in practice; though with the exquisite workman ship of modern times, we think it might be resum ed with considerable probability of success.
When Mr. Wren was at Oxford, he was employ ed by the king to make drawings of animalculze seen by the microscope; and such was their merit, that Dr. Hooke, in his preface to his Micrographia, states, that he began it with reluctance, as he had to follow the footsteps of so eminent a person as Dr. Wren, who was the first that attempted any thing of this nature, and whose original draughts made one of the ornaments of the great collection of rarities in the king's closet. He then adds, " 1 must affirm of him, that since the time of Archi medes there scarce were met in one man so great a perfection—such a mechanical head, and so philo sophical a mind." Sir Isaac Newton speaks of Sir Christopher with equal praise, and names him along with Wallis and Huygens, as one the first mathe maticians of the age.
Such were the scientific labours of this eminent individual, but though he did not abandon his early studies, yet his mind was now directed to new ob jects, which, while they principally occupied his mind, conducted him to fame and honours if not to fortune.