HISTORY OF PHYSICAL ASTRONOMY.

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consequence, the appellation of the Triduum . Unfortunately, it con-tains the only results of his labours that have been preserved, all his otherobservations having been destroyed during the fire of 1728. In accuracythey are surpassed only in a slight degree by the observations of the pre-sent day. The passages across the meridian are generally observed at thethree vertical threads, or wires of the focus, as they are now technicallytermed. The time is in several instances assigned to fractions of a second.It is no mean tribute to the memory of Roemer, that the observations ofthe Triduum constitute the earliest collection of facts that have beendeemed worthy of being employed as data in the solution of the greatproblem of modern astronomy—the motion of the Solar System in space.

The Tusculan Observatory was built in the year 1704, and in the monthof December of the same year, Roemer commenced his labours in it,which he continued to prosecute till his death in 1710 *. TIorrebow statesthat the observations made by Roemer during this period, filled three largefolio volumes, and equalled in number those of Tycho Brahe . The ob-servations were continued after the death of Roemer, till the autumnof 1711, when the Observatory , by some unexplained casualty, was de-stroyed The instruments, which sustained great injury on this occasion,were transported to the ancient Astronomical Tower , where, with all theobservations, except the Triduum of Roemer , a copy of which was safelydeposited in the hall of the Academy, they were finally consumed by thedreadful fire of 1728.

Roemer was the first person who constructed an altitude and azimuthcircle. Tycho Brahe and Hevelius had made use of quadrants moveableabout a vertical axis, but Roemer did not fail to perceive that such instru-ments were essentially defective. The diameter of the vertical circle ofRoemer’s instrument was three feet five inches ; the diameter of the azi-muth circle was three feet eight inches. The instrument was chiefly used

betrayed into error upon any subject, when, instead of carefully examining facts, heallows his mind to be so wholly engrossed by his own favourite ideas as to look for nothingelse except what may appear to corroborate them. We venture to assert, that the wordsabove cited are not to be found anywhere in the book which Delambre professes to beanalyzing. Indeed, it is obvious that they are utterly inconsistent with Horrebow’s de-scription of the Tusculan Observatory , the edifice to which they relate. Delambre him-self has not failed, in the above passage, to remark that the regret expressed by Horrebowis at variance with the fact of several circumpolar stars having been observed by Roemer;but it seems wholly unaccountable that he was not thereby induced to look more nar-rowly into the subject, so as to discover the origin of this apparent contradiction. It isnot difficult to trace the passage in the Basis Astronomies which Delambre has so griev-ously misinterpreted (see p. 151 of the work just cited), but it is manifest that only themost unwary explorer of that work, seduced by some mental hallucination, could havebeen ensnared by such a passage.

* Delambre remarks, that as the observations which form the basis of Roemer’sfragment on the annual parallax of the terrestrial orbit, inserted by Horrebow inhis Basis Astronomies, were made by Roemer in the year 1692, they were anterior to theerection of the Tusculan Observatory , and he therefore concludes that we are totally ig-norant of the kind of instrument employed by Roemer in these observations. (Ainsinous ne savons pas quel instrument il a employe dans ces observations. Hist. Ast. Mod.,tom. ii., p. 640.) This is another striking instance of the mistakes into which Delambrehas fallen by confounding the Observatorium Domesticum of Roemer with the Observa-torium Tusculaneum. The observations referred to were, in fact, made at the former ofthese edifices, (which was fitted up as early as 1690,) with the original transit instrument, orMachina Domestica, already described in the text.

+ Horrebow simply says —“ Postea desolatum est observatorium.” From this expression,and also that which follows, “ atque inslrumenta jam magnis ictibus fracla,” it wouldseem that the building was overthrown by the violence of some dreadful storm.