therefore, which measures the atmospheric pressure, is so comparatively small, that barometrical instruments constructed with it are not of an inconvenient size. If water were used, it is evident that the column would require to be some 30 feet or less* in height (supposing the average height of the mercurial column to be 30 inches, and seeing that the specific gravity of mercury is 13.575, it follows that the height of the corresponding water column would be 30×13'575=407.25 inches, or 33'9 feet). Though the average height of the mercurial column is taken at 30 inches, it is found to fluctuate between 28 and 31 inches at the level of the sea; but these extremes are very rarely reached, and it is because its general height does not vary much from 30 inches that this has been selected by English writers as the standard height.† When mercury is at a temperature of 32° F., the weight of one cubic inch is 0'491 lbs. From this we can calculate how many pounds on the square inch the pressure of an atmosphere corresponding to a barometric column of 30 inches is equal to. Let us suppose for the sake of simplicity that the area of the cross section of the mercurial column is I square inch; the atmospheric pressure will then be equal to 30x0491=1473 lbs., or, in round numbers, 15 lbs. to the square inch. When the elastic force of a gas or vapour is very considerable, it is usual to estimate it in so many atmospheres; thus, when steam is said to have a pressure of 5 atmospheres, it means that its elastic force is such that it would sustain a column exerting a pressure of 73.6 lbs. (1473x5=7365) on the square inch. There is one phenomenon observed in connexion with mercurial barometers, which from its singularity must be noticed. In many cases where the mercury is shaken, a luminous appearance becomes visible in the vacuum; this is termed "the Barometrical Light." Sometimes it is diffused all over the vacuum; at others it is confined to near the surface of the mercury. Its nature is unknown; but it is presumed to be electrical, though no satisfactory reason has been assigned for its appearing in some barometers and not in others, and for its appearing and disappearing from time to time in the same instrument. It was first noticed by Picard, and subsequently by Cassini, La Hire, &c.‡ The history of the circumstances that led to the discovery of the barometer is of peculiar interest. It appears that the constructors of a pump made for the Duke of Florence found them *Practically less than 28 feet, owing to air and mechanical difficulties.— (Fitz-Roy) Weather-Book, p. 12. + For London, Admiral Fitz-Roy assumes it to be 29.95 inches.—Ibid. P. 15. De Luc, Recherches sur les Modifications de l'Atmosphere. selves unable to make the water rise higher than 32 feet or thereabouts, when the air was exhausted from it. They applied to the celebrated philosopher Galileo for an explanation, and he contented himself by repeating the Aristotelian dogma that "nature abhorred a vacuum.' This reply satisfied the pumpmakers, though it did not, as far as we can tell, altogether satisfy Galileo, and there is reason to believe that he abandoned the idea before his death. It is, however, to his pupil Torricelli that we owe the first public acknowledgment that the phenomenon would be correctly explained by supposing it due to the counterbalancing of the 32 feet of water by the pressure of the atmosphere. He saw that if it be a weight of air which counterpoises 32 feet of water, then it must follow that, by the substitution of mercury for water, in consequence of its greater specific gravity, a column shorter than 32 feet, in the approximate ratio of 14: 1, would answer equally well. Torricelli tried the experiment, and found that a column of mercury 2 feet 4 inches in height would counterpoise the pressure of the atmosphere, thus proving beyond a doubt that the explanation he had given was the true one. * Torricelli died in 1647, leaving his discovery not quite complete; for though he had ascertained that the weight of the water and the weight of the mercury was a counterpoise of something, most probably the weight of the air, yet that was not absolutely certain. The subject was taken up in England by Boyle, and in France by Pascal, Mersenne, and others. The former, by means of the air-pump, was enabled to try the barometer with air of different degrees of density. Pascal in another way effected the same object. In 1647 he suggested the idea that if the mercury were really sustained by the pressure of the atmosphere, it would necessarily fall in ascending a high mountain. Though illness prevented Pascal himself from personally establishing this fact, his relative, Perrier, undertook to do it for him. Accordingly, an expedition, headed by the latter, started on Sept. 19, 1648, to ascend the Puy-de-Dôme, in Auvergne. They found the result precisely as Pascal had foretold, and this finally settled the question. Similar experiments were carried on in England soon afterwards, and with the same results. * Vide Montucla, Hist. Mathemat. vol. ii. p. 203. Table exhibiting the Corrections to be subtracted from the Observed Height of the Barometer to reduce it to the Temperature of 32° Fahrenheit. Ther. Inch Inch Inch Inch Inch Inch Inch Inch Inch *016 *016 42 0.019 0'020 O'021 0'022 0023 *026 50 *035 *037 00240025 0.025 0.026 '027 *028 030 '030 '031 '031 032 *035 036 *038 039 *040 *041 *043 *045 *046 *048 *033 *035 *035 *037 *040 '041 '042 54 *047 *049 '051 053 60 *055 *057 0.039 0.041 0043 0.044 0.046 0.048 0050 0·051 0053 *043 *045 *047 *048 *050 *052 *055 *056 *058 *052 053 *055 *057 обо 061 *063 *056 *057 059 *065 *067 068 *060 *062 064 067 *070 *072 *074 62 0059 0.061 0.064 0.066 0.072 0.075 0.077 0.080 64 063 *065 *068 *070 073 *076 *079 *082 *085 *067 *069 *072 '075 '071 *073 '077 *079 70 '075 *078 *081 *084 *087 0079 0082 0.085 0.088 0.092 0.096 0.099 0.103 0106 *086 *089 *093 *096 '100 '104 *108 'III *090 '094 *097 ΙΟΙ *105 *109 113 *117 *094 *098 '102 *105 'I10 114 118 *122 *099 '102 *106 '110 *115 *119 123 *128 *164 '170 *141 *146 152 *156 *163 *169 *176 *146 '151 *157 •161 168 174 *181 102 0138 0144 0150 0155 0161 0166 0173 0179 0186 104 '142 *148 *154 *159 *165 '171 '178 *184 *191 106 *146 *152 159 164 *170 *176 183 '190 *197 108 *150 *157 *163 *169 *175 •181 *189 '154 •161 *168 *174 •180 *187 112 0158 0166 0172 0178 0185 0192 0199 0206 0213 *195 *202 *194 '201 *207 CORRESPONDENCE. N.B.-We do not hold ourselves answerable for any opinions expressed by our correspondents. T. CORONE. TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-As several of my correspondents have been puzzled by a mistake on the part of the engraver of the chart for the new star in Coronæ, published in the August number of the Register, will you allow me to explain that the large star, 43 mag., in the lower part of the chart, is e epsilon) Coronæ? The engraver has turned the round and made it look like a 3. The chart, which is, with the exception of the addition of one star, merely brought up from Argelander, represents the field as seen in an inverting telescope. I am, Sir, yours faithfully, Woodcroft Observatory, Cuckfield: GEORGE KNOTT. CATALOGUE OF RED STARS. TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-During the whole course of my acquaintance with the astronomers of your little planet, now extending over several thousand years, I have always understood from them that theirs was the most exact of sciences, and one to which accuracy of diction was especially important; and I was not aware of any want of unanimity in this opinion until the appearance of Mr. Chambers's letter in the last number of the Astronomical Register. He therein declares that the list of red stars which he appends contains "all stars above the 7 magnitude inclusive;" from which assertion it appears that as every star in the catalogue is "above the 7 magnitude," and therefore that No. 1 exceeds that magnitude, and as every reader of the Register must grant 75 equal to 7, it follows hence that No. 1 is equal to and also greater than itself a statement which, according to an ancient friend of mine, yclept Euclid, is absurd; and it is therefore due to the readers of your periodical to explain which part of his assertion he wishes to sustain. It is also due to myself that some explanation should be given of the intimation, contained in the same communication, that I have abdicated my position in favour of Aldebaran! And in the meanwhile, I am, Sir, where I have always been, September 15, 1866. Yours faithfully, ARCTURUS. [We have referred this letter to the gentleman concerned, who does not think it worth a reply.] MEETING OF THE BRITISH ASSOCIATION. TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,--One of the most interesting papers read at the meeting of the British Association, at Nottingham, was by J. R. Hind, Esq., F. R.S., on the variable star lately discovered in Corona Borealis. It appears from a correspondence which Mr. Hind has carried on with Mr. Barker, of London, Canada West, that Mr. Barker first saw the star on the 4th of May 1866, at 9 P.M., when it was somewhat brighter than e Coronæ ; it rapidly increased until the 10th, when it was fully as bright as a Coronæ, and at its maximum. The appearance on the 12th has been already given in your pages. On the 14th, Mr. Barker observed it of the 3rd magnitude; on the 18th, of the 5th; on the 19th he could just discern it; and on the 20th he could see it no longer with unaided vision. Mr. Barker's earliest announcement of this remarkable star was on the 16th of May, as follows:: "Astronomers will be interested to learn that a new star has made its appearance in the constellation of Coronæ Borealis. It is of the 3rd magnitude, and is situated about one degree S.E. by E. of Epsilon Coronæ, and three degrees from Pi Ophiuchus." Mr. Hind has rendered great service in bringing these earlier observations before the British Association. He very justly observes that they are of historical and scientific value-first, as showing the date of discovery; and, secondly, the increase from a smaller to a greater magnitude previous to the star being recognised in Europe, which places it in the category of ordinary variable stars, the apex of the light curve being very acuminated, and probably of long period. At the lecture by Mr. Huggins, on the evening of the day on which Mr. Hind's paper was read, the spectrum of the star was exhibited. It would appear, from the great number of absorption bands, that the ordinary light of the star is feeble, not, perhaps, on account of its distance, but rather because its light is absorbed by its atmosphere. The four bright gaseous bands were very decided, and these doubtless contributed to the attainment of the maximum. In the course of the remarks which followed the reading of the paper, Algol was mentioned. It is not unlikely that the spectra of Algol at maximum and minimum might throw some light on this remarkable outbreak. I am, Sir, your obedient servant, Nottingham: August 25, 1866. W. R. BIRT. THE HUNTER'S MOON. TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-" Q. C.," in your last number, p. 247, asks, "Which is right-Arago, who says the full moon nearest the Vernal Equinox is the Hunter's Moon; or Smyth, who says it is the one which follows the Harvest Moon?" I beg leave to place Ferguson in the witness-box, and to remain, Sir, Your obedient servant, September 7, 1866, W. R. BIRT. |
