will probably be found lower than those of any other substance, and it appears to possess higher acidifying and saturating powers than either oxygen or chlorine.

It is easy to perceive, in following the above theory, that all the ideas current in chemical authors respecting the fluoric combinations, must be changed. Fluor spar, and other analogous substances, for instance, must be regarded as binary compounds of metals and fluorine.

Many objects of enquiry arise, likewise, from these new views: the topaz contains the fluoric principle, but new experiments are required to shew whether that gem is a true silicated fluate of alumina, or a compound of the inflammable bases of alumina and silica with fluorine.

I have ascertained that the chryolite yields no silicated fluoric gas, when acted on by sulphuric acid, but merely pure fluoric acid; but I have not continued the research so far, as to determine whether it contains fluorine united to inflammable matter only, or fluorine and oxygen.

3

XXXII. Catalogue of North Polar Distances of Eighty-four
principal fixed Stars, deduced from Observations made with the
Mural Circle at the Royal Observatory. By John Pond, Esq.
Astronomer Royal, F. R. S.

Read July 8, 1818.

THE Catalogue, which I have the honour to transmit to the Society, is deduced from the whole of the observations made with the mural circle, from its first erection in June, 1812, to the present time. I am still employed in endeavouring to give it a greater degree of precision, and when it is entirely finished, I propose to submit some of the observations themselves to the Society, and explain the method by which the results have been obtained.

I have already mentioned, that I use neither level nor plumbline; but determine the position of the instrument by means of a standard catalogue of stars derived from the instrument itself, in such a manner, that every series of observation of these stars, serves the double purpose of ascertaining the posisition of the instrument, and at the same time improving the Catalogue.

As the present Catalogue has been formed by frequently changing the position of the telescope on the circle, for the purpose of correcting every possible error of division, my observations have not been calculated to decide the question of parallax which has been suspected to exist in a Lyræ, a

α

Aquilæ, and some other stars. But from this time forward, I propose to use the instrument in one position of the telescope, with the hope of ascertaining, if not the parallax of these stars, at least the limits, which it does not exceed.

Though the change of position in the telescope, by which all error of division is avoided, is one of the most beautiful properties of this instrument, yet so accurately is it divided, that I cannot perceive, with certainty, any effect produced by this change, for I have often found as great a discordance between two series of observations made on the same divisions, as when they are entirely changed by a new position of the telescope. What the error of division may amount to in any one position, I cannot exactly say; but, I think, when the six microscopes are used, it can never exceed half a second, and very rarely amounts to half that quantity.

That some opinion may be formed of the accuracy of this instrument, I have subjoined to the Catalogue the results of the observations of some of the standard stars, whose places I am anxious to determine with the greatest precision, since I propose in future to determine all north polar distances by comparison with these stars; precisely in the same manner as right ascensions are now determined by comparison with the thirty-six stars, whose places have been so accurately determined by Dr. MASKELYNE.