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HISTORY OF PHYSICAL ASTRONOMY.

vatory on a permanent basis. Longomontanus, an astronomer of somecelebrity, wlio had assisted Tyclio in liis labours at Uranibourg , was chieflyinstrumental in promoting the views of the monarch. The foundationstone of the observatory was laid by Christian IV. , on the 7th of July,1637, but it does not appear that the building was completely finisheduntil the year 1656, during the reign of his successor, Frederick IV .The structure was erected in the form of a tower. It was 115 Danishfeet in height, and 48 feet in diameter. The observations were made inan apartment at the top of the tower, which was reached by means of aspiral avenue winding gently round the interior of the wall*. We shallpresently have occasion to allude more particularly to this observatory,which must be regarded as the earliest of the numerous public institutionsfor the promotion of practical astronomy that have been established inmodern times.

It has been already mentioned that all contrivances for measuring timeby means of clocks composed of a combination of wheels moved byweights, had been found totally inadequate to the delicate purposes ofastronomy. Another mode of effecting the same object had been devisedby Galileo. Having discovered that the oscillations of a pendulum areisochronous f, it occurred to him that a mechanism might be constructedupon this principle which would serve to indicate the divisions of timewith great accuracy; but although he devoted much attention to the real-isation of this idea, all his efforts to procure a clock that would be practicallyserviceable proved fruitless. In all such contrivances it was found thatthe pendulum, when once set in motion, was soon again brought to a stateof rest, so that the oscillations could only be maintained by applying tothe pendulum a succession of impulses at regular intervals. Huyghens eluded this inconvenience, by substituting the pendulum for the balancein the ancient clocks, so adapting it, that the isochronism of its oscillationsregulated the downward motion of the weight. By this admirable con-trivance he combined the advantages of the two methods above referredto, while, at the same time, he eluded their respective defects. The ideaof so applying the pendulum to the measurement of time first suggesteditself to him in the year 1650, and in the following year he presented to theStates General of Holland a clock constructed upon this jirinciple. Theinvention soon spread abroad throughout Europe , and was hailed with uni-versal applause. Its successful application to the purposes of astronomicalobservation will be noticed presently.

The establishment of the Royal Society of London, and of the Academyof Sciences of Paris , exercised a powerful influence on the progress ofastronomy, as well as on every branch of physical science. The origin ofthe Royal Society may be traced to the period of the civil wars in the timeof Charles I. , when individuals of a contemplative cast of mind soughtrelief from the prevailing commotion in the calm retreat of scientific dis-cussion. Dr. Wallis states that, about the year 1645, when academicalstudies were much disturbed at both the universities, several personsinquisitive into natural philosophy, and other parts of human learning, did,

* Horrebow, in his Basis Astronomies, states that in 1716 Peter the Great ofRussia frequently ascended the tower on horseback along this spiral avenue, and that theEmpress Catherine occasionally ascended at his side in a four-wheeled chariot, drawn bysix horses!

f They are not exactly isochronous, but for small arcs the times of oscillation maybe regarded as equal without committing any sensible error.