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3. Why does the "Apparent R.A. at transit" differ from that "at mean

noon?"

I suppose I ought to be able to answer this for myself; but as I cannot do so satisfactorily, I prefer to lay out an anchor for a little instruction. If you feel disposed to enter more fully into the matter, you might devote a few lines to telling us,

4. To what use can we put col. 4, “Log. of True Dist. from the Earth?" 5. In what cases are the heliocentric measurements of use?

And lastly, what is the meaning of the "o" standing at the head of columns 4 and 8? Is it the "characteristic" of the logarithm whose "mantissa " is subjoined ?

Oct. 17, 1866.

I am, &c.,

DOUBLE STAR NEAR ALBIREO.

ATHLETA.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-Your correspondent Mr. Jones (Astronomical Register, November, p. 292) puts a query which seems difficult to answer, and I cannot but suspect some misprinting. He speaks of a star situate "between Albireo and y Lyræ- about 20 from the former, and 1° south of it." Now, a glance at the map will serve to show that no object whose declination is south of Albireo can possibly be in, or nearly in, a line between it and y Lyræ at all. It follows that there must be some mistake. If your correspondent is not provided with a graduated instrument, any tolerably exact description by way of alignment would easily procure him all the satisfaction he requires. I remain, Sir, yours faithfully, R. J. GOULD.

Reading: Nov. 5, 1866.

STELLAR SPECTROMETRY.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-The word "lines" referred to by "Spectrum Analysis," at p. 290 of your last number, was a lapsus calami on my part for "stars."

Secchi describes the prism in a letter on the Spectrum of Antares, in the recently published Notice of the Royal Astronomical Society, where he states that it is of strong dispersive power, and not of low dispersive power as suggested by "Spectrum Analysis."

Yours obediently,

THE TRANSLATOR OF SECCHI'S ARTICLES.

November 15, 1866.

MASS OF THE PLANET JUPITER.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-When I had read last night a paper at the meeting of the Royal Astronomical Society, in which I offered an account of a late investigation by Herr Krüger, of Helsingfors, into the mass of the planet Jupiter, with a short history of previous determinations, some gentleman, with whose name I am not acquainted, but which you may, perhaps, be able to supply, and who spoke of himself as a friend of the late lamented Captain Jacob, expressed his regret that I had not noticed the value determined by that astronomer from the motions of the satellites at the Madras Observatory. I the more regret this myself that Krüger, in the paper from which I

quoted, does mention Captain Jacob's determination as confirmatory of those of Airy and Bessel. Nevertheless, as the observations were made several years afterwards (all in the latter part of 1857), and, as the Captain observes (Memoirs R. A. S., vol. xxviii. p. 110), principally "to furnish a test by which to judge of the value to be assigned to those of the Saturnian system made by the same observer with the same instrument," I did not conceive them essential to my paper, which I wished to make as short as was consistent with its object. All Captain Jacob's observations of the satellites were made by himself, and the result he obtained for the mass of Jupiter was 1647-54, being between Airy's and Bessel's, and very close indeed to the recent one of Krüger.

Greenwich: Nov. 10, 1866.

I am, Sir, yours faithfully,

W. T. LYNN.

[The gentleman was Major Tennant. See report of the meeting.-Ed.]

THE BAROMETER.

BY G. F. CHAMBERS, F.R.A.S.
(Continued from page 289.)

Ir may be observed that the foregoing is what may be termed the strictly scientific barometric method of determining the heights of mountains, but that others, involving the use of much less cumbersome apparatus (an immense desideratum to the traveller), have been introduced of late years. The hypsometrical plan is one of the best. The boiling point of water is commonly assumed to be 212° at the level of the sea, but at any elevation above this level the boiling point falls below 212°; and as the elevation increases, the thermometric depression increases, in consequence of the lessening of the atmospheric pressure. The practical application of this fact is the basis of the hypsometrical method of determining altitudes. The temperature of the vapour of the boiling water and of the air, at the lower and the upper stations, is ascertained with a thermometer, and then a short calculation performed with the aid of two simple tables gives the result (and a very accurate one, too) without trouble or delay. Casella supplies a compact form of apparatus, so portable that it may be carried in the pocket.

To determine the difference of altitude of two stations, it is simply necessary to observe the temperature of the vapour of the boiling water and of the air at the lower station, and the same at the upper station. Then Table I. gives by inspection the approximate altitude of each station, whence the difference may be obtained by simple subtraction. Table II. furnishes a multiplier for correcting these results for difference of temperature of the circumjacent air. The multiplier for the mean temperature is to be taken.*

* These tables are adapted from those given by Col. Sykes, F.R.S., and Col. Shortrede.

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Then, multiplier for 54 is (from Table II.) 1·049.

9850 × 1049:

=

10333, the corrected difference of the altitudes of the two stations.

Approxi- Value of

level of the

TABLE I.

of baro

meter

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Boiling point of pure water

each tenth of a

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ing height

degree

in feet

sea (or 30'00 in.)

of alti

tude

sea (or 30'00 in.)

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It may assist your "Subscriber," in his estimate of his 5-inch objectglass, to inform him that my own 4.2-inch glass, by the late Andrew Ross, with a power of 394, turns 72 Andromeda into an egg-shaped object, though certainly without separating the components; but that with the same power, on a fine night, I have seen the companion of 8 Cygni as a faint green dot interrupting the rings round the larger star, and fairly divorced from it. ANOTHER SUBSCRIBER.

In answer to "A Subscriber," in your November number, I beg to say that my 8-inch object-glass divides y2 Andromedæ. easily; also when same object-glass is diminished to 8, 7, 7, 6, and sometimes 6 in., but only in very fine nights. I don't think I ever saw the small star with a less aperture than 6 in.; certainly 5 in. will not show it in my equatorial.

S.

For the information of your correspondent "Curiosus," I have to state that the next total solar eclipse visible in Europe will occur on December 22. 1870. The shadow enters the extreme south of Portugal, thence into Andalusia near San Lucar, and, passing a little to the north of Cadiz and Gibraltar, crosses over to near Oran, in Algeria. At San Lucar it will be nearly central about noon.

From the lesser difference between the solar and lunar diameters, its duration will, I presume, be shorter than that we saw in Spain in 1860, but the time and place are favourable.

The eclipse of March 5th, 1867, will be annular about Salerno, in Italy; that of August 19, 1887, occurs at 6 A.M. W. L. BANKS. eclipse of the sun 1867 as passing G. J. W.

As some reply to "Curiosus," the time of central (annular) is represented in the "Nautical Almanac" for through Algiers and the South of Italy March 5-6.

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ALTHOUGH many of the greatest discoveries have been made by reflecting telescopes; although they are identified with the labours of the Herschels, Lassell, De la Rue, Nasmyth, and other great workers in the cause of astronomy; and although it is known that they are to a great extent free from the disturbing effects of colour hitherto inseparable from the refracting instrument, it cannot be concealed that they have of late years met with but little favour. At one time certainly small reflectors were more used, and those who are familiar with the works of Dr. Pearson and Dr. Kitchener know that opinions were divided as to the merits of the mirror and the object glass, with perhaps some preponderance in favour of the former. But at that time object glasses were produced of comparatively small size only, and few in number. Now that our modern English makers have succeeded not only in constructing large instruments, but mounting them in that solid and effective manner which is to be met with in so many private

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