True Apparent Places of Thirty-seven of the Principal Fixed Stars for every Tenth Day of the Year. Jan. 1 20 29.16 6 20.4 28 10.24 40 0.9 31 59.43 38 56.7 18 9.07 49 37.2 43 40.38 29 11.6 Day of the Month. True Apparent Places of Thirty-seven of the Principal Fixed Stars for every Tenth Day of the Year. -The Upper Culmination at Greenwich. Epoch. "1 m. S. h. 22 m. S. 22 14 m. S. Jan. 1 36 26.90 45 49.5 15 3.83 58 23.1 58 18.40 1 30.1 49 36.35 23 39.5 57 30.97 25 30.4 3.62 20.4 18.36 30.9 36.27 39.1 30.88 29.3 2 a Pegasi (Markab). 1 Right Asc. Dec. North. Dr. Young's Refractions, the Barometer being at 30 inches, and the internal Thermometer at 50, or the external at 47 degrees; with the Corrections for +one inch in the Barometer, and for one degree in the Thermometer of Fahrenheit. From page 19 of Vol. I. of Pearson's Practical Astronomy. - The correction for an increase of altitude of one inch in the barometer, or for a depression of one degree in the thermometer, is to be added to the tabular refraction; but when the parometer is lower than 30 inches, or the thermometer higher than 47 degrees, the correction becomes subtractive. When great accuracy is required, 0.003 inch should be deducted from the observed height of the barometer for each degree that the thermometer near it is above 50 degrees, and the same quantity added for an equal depression. Logarithm for converting Sidereal into Mean Solar Time +9.9988126. Mean Solar into Sidereal Time +0.0011874. A second of time at the Equator contains 1521 feet. ATMOSPHERICAL ELECTRICITY. By Professor Joseph Lovering of Harvard University. In the last volume of this Almanac, I gave an exposition of the electrical states of the earth and its atmosphere; of the ways in which these electrical states are produced, and the various methods contrived for studying them. I propose in the present or the next volume to add some remarks on the physical cause of thunder and lightning, considered as the visible tokens of electric discharge; on the danger of being struck by lightning; on the protections against it, natural and artificial, which have been provided by Providence or human agency; on lightning-rods, and their efficacy; and on the effects which electricity may possibly produce when and where it chances to strike. Aristotle speaks thus of the phenomena to be discussed: "We, how ever, say that the same nature upon the earth is wind, but in the earth is an earthquake, and in the clouds is thunder."* Aragot has defined thunder and lightning as a phenomenon or meteor which is exhibited when the heavens are covered with clouds; and which manifests itself first by light and then by noise. I will not dwell upon the fanciful distinctions on this subject made by the Etruscans, renowned as they were in the ancient world for their knowledge of these things. Pliny divided lightnings into public and private. He also distinguished between those which came from the stars and those which rise out of the earth. But Aristophanes, in the Clouds, ridicules the idea that thunder ever comes from the earth. Regarding thunder and lightning as an atmospheric phenomenon, revealing to man's senses the violent discharge of electricity between one cloud and another, or between the earth and a cloud, I proceed first to inquire into the peculiarity and height of thunder-clouds. Arago mentions, as one peculiarity, a kind of fermentation, which Forster compared to that of cheese when full of maggots. Peytier and Hossard, while engineering upon the Pyrenees, observed that, even when the clouds were smooth underneath, they were often rough above. The effects of the lightning's stroke have been found on the highest mountains. Humboldt recognized them in South America; Saussure discovered them on Mont Blanc; Ramond, Peytier, and Hossard met them on the summits of the Pyrenees; and Bouguer and Condamine on the Cordilleras. But it is not safe to presume that thunder-clouds reach as high as the effects of their explosion. For the lightning may strike from a lower cloud up to a higher peak as well as downward. There is a church in Styria, standing upon a prominent mountain-top. On May-day, 1700, a physician at that place noticed a dense black cloud below him; the sky above was a clear blue, when a flash of lightning ascended from the cloud, struck the church, and killed seven persons. Murray says he has seen lightning ascend in a spiral line. But even if it were admitted that the height of thunder-clouds is coequal with the marks of lightning, it would still be necessary to inquire whether thunder-clouds rise to that height in level countries. To answer this question, resort is had to an observation of the interval which elapses between the flash and the report. But unless the angular elevation of the cloud above the horizon is recorded, and is taken into the account, the observation gives, and can only give, the distance of the cloud from the observer, and not its perpendicular elevation above the earth's surface. And this distance is correct only on the supposition that the sound is made in the cloud, and not along the whole line of discharge. Subject to these corrections, a large number of illustrative cases, compiled by Arago, may * V. 535. † Annuaire, 1838. |