ILLUSTRATED ASTRONOMY. SSON XLI. TIDES. have the earth and moon, besides revolving e around their common centre of ght line joining their centres, is the centre s from the centre of the earth. centrifugal force upon the water on the om the moon? ecede from the centre of gravity, -t of the earth. pon the shape of the earth? lengthened in the line of the shortened at right angles to it. e this oval shape of the earth? f the attraction of the moon at he earth. of the earth nearest to the moon is more the earth; the water upon the opposite urning of the earth, from west to east on Q. Does the water remain permanently higher on the east than on the west side of the continents? A. The gulf of Mexico is 20 feet higher than the levations, or tide waves, to pass Pacific ocean, and the Red sea is 30 feet higher than ad the earth.. ind falling of the waters of the ded with respect to the rising and falling the water. the greatest elevation of the flood tide? the water. 1 ebb tide occur? hours. i same hour every day? 1 hour later each day. ater? passes the meridian about an to be later at the meridian? revolving monthly around the he sun produce an effect similar to that tide two fifths as high. on are on the same or opposite sides of their attractive forces? equal to the sum of their sep rature, what is the effect of their coun qual to the difference of their the Mediterranean. Q. Where the tide wave is least obstructed, as in the Pacific ocean, how much behind the moon is it? A. It is two or three hours behind it. Q. How long after the moon passes the meridian, is it high water at New York? A. About 8 hours. Q. If the earth were uniformly covered with water, how high would the tide rise? A. Not more than two or three feet. (The tide at the small islands in the Pacific ocean is usually less.) Q. What produces the greatest effect in causing high tides? A. The shape of the land, and the position of the shores. Q. Where are the highest tides in the world? Q. What, besides the position of the shores, tends to raise a high tide at this place? A. The meeting of the tide wave from the North Atlantic ocean, with the main one from the South Atlantic. Q. How high are the average spring tides at Cumberland, ear the head of the Bay of Fundy? A. About 71 feet. Q. How high are they at Boston? A. About 11 feet. Q. At New York? A. About 5 feet. Q. At Charleston, South Carolina? A. About 6 feet. Q. When do we have the highest tides in the northern hemisphere? A. At new moon in the summer, and at full moon in the winter. (See Diagram.) OP ILLUSTRATED ASTRONOMY. ON XLIII.: PLANETS AND COMETS. it of a primary planet? h in which it revolves around secondary planet? · A. Some are entirely destitute of any such appendage. A. The number is not known; about 500 have been Q. Are comets solid bodies like the planets? A. They generally are not; although some have been which it revolves around its observed to have a dense nucleus, or head. ? Q. What is the nature of comets? A. They are supposed to be gaseous matter, in the Q. Do comets shine by their own, or by reflected light? Q. Do they all, like the planets, revolve in the same direction A. They do not; they revolve in different directions. A. They are not; their orbits are in all directions in Q. How do many of them move when first seen A. They appear to move in almost a direct line Q. Does their velocity increase as they approach the sun? Q. How fast has a comet been known to move? A. 880,000 miles an hour. Comets. COMETS were anciently viewed by mankind with astonishment and de: eight degrees on each side fear, as being forerunners of dreadful calamities, such as war, famine, 'e around the sun in the same direction? st to east. he same velocity? sun. or pestilence. Many ancient philosophers considered them as only meteors in the atmosphere. Tycho Brahe was the first who showed that they belonged to the planetary system, and revolved around the The orbits of all the comets are very elliptical, so that they ases as the distance from the approach the sun almost in a direct line, and after being involved in the light of the sun for a short time, depart from our solar system in nearly the same direction in which they approached, and remain for years, or even centuries, beyond the limit of the best telescopes. its orbit with the greatest velocity? cast? north latitude? r north of the earth's orbit. south latitude? r south of the earth's orbit. Very little is known of the physical nature of comets; the smaller comets, such as are visible only with telescopes, generally have no appearance of a tail, and appear like round or somewhat oval, vaporous masses, more dense towards the centre; yet they have no distinct nucleus or solid body. Stars of the smallest magnitude are seen through the most dense parts of these bodies. It is very probable that the luminous part of a comet is something of the nature of smoke, fog, or other gaseous matter. Halley's comet, which appeared in 1456, with a tail 60 degrees in length, and spread out like a fan, has appeared periodically every 77th year, viz.: 1682, 1759, and in 1836; but it has exhibited no tail, or luminous appendage, since 1456. The comet which appeared 371 years before Christ, is said to have covered a third part of the visible heavens. A remarkable comet måde its appearance 48 years before Christ, and was so bright as to be visible in the day time; it was supposed, by the superstitious, to be the ghost or tail, on the opposite side of Caesar, who had just been assassinated. The following are some ON XLIV. METS. CROWN AL ARCHER SHIELD ILLUSTRATED ASTRONOMY. COMETS.-Continued. AT are the principal parts of a comet? - Nucleus ? most dense or solid portion, sometimes 1. (See the comet of 371.) e Envelope? minous matter surrounding the Nucleus. Tail of a comet ? ong luminous train extending off from e opposite direction from the sun. (See has the eccentricity of their orbits upon the motion See Note 2.) tion increases as they approach the sun as they recede from the sun. has the change of position upon their appearance? ls usually increase both in length and NOTES. NOTE 1.-Comets assume a great variety of shapes; some appearing like an erormous fan, others like a long sword or sabre; but all curved more or less, and concave towards the region from which they come. The Comet of 1744, represented on the opposite page, excited great attention. NOTE 2.-The orbits of Comets are very elongated, having their perihelion very near the Sun; (see diagram, page 45,) consequently as they approach the Sun their velocity increases rapidly by the increased attraction of the Sun, and when at their perihelion they move with immense velocity. NOTE 3.-The Comet of 1680, came within 130,100 miles of the Sun, and must have received 28,000 times more light and heat than the earth receives from the Sun. Sir Isaac Newton calculated the heat of this Comet to be 2,000 times greater than red hot iron, and that it would require 2,000 years to cool; he assumed that the Comet was a solid body, which was not the fact. It is a generally conceded fact at the present day that the rays of the Sun to produce much heat, must come in contact with solid bodies; and as Comets are of an extremely thin gaseous matter, the rays of the Sun may pass through them without producing much heat, this is more probably the case. We find in ascending high mountains that the atmosphere becomes very cold which ought not to be the case if the rays of the Sun impart much heat to the atmosphere in their passage through it. It is only when the rays of the Sun come in contact with the earth that much heat is produced. The density of comets is probably not so great as that of our atmosphere and as they have no solid Nucleus or heads, the probability is that comparatively very little heat is produced by their near approach to the Sun. NOTE 4.-The Comet of 1811, was regarded by the ignorant as the precursor of the War that was declared in the following spring between Great Britain and the United States. In some cases Comets have excited fears that the day of judgment was at y approach the sun and contract as they hand, and that the comet was sent to burn up the world. In 1773, de Lalande of e sun. known of their Temperature? st be very hot when near the sun. (See ou say of the size of comets? Paris announced to the Academy that there was great danger of the Comet which iclei or heads are usually small, being phere. It is very probable that comets contain no aqueous vapors; but are simply of ɔ 2000 miles in diameter. omets revolve around the Sun continually? opinion of Astronomers that all the to our Solar System, but they are not nounce it as a scientific discovery. Sir John Herschel are of opinion that the greater number visit nd then fly off in nearly straight lines till they pass the center of Solar System and Fixed Stars, and go to revolve around suns in omets regarded by the ancients? ngers of famine, pestilence, war, and nities. (Note 4.) fears have been entertained of comets? a gaseous matter, and it does not follow from this that it would produce any evil con- HAS THE EARTH PASSED THROUGH THE TAILS OF COMETS? ' might come in collision with our globe and was entirely destitute of any moisture, whereas most fogs are moist; besides all be the effect if a comet should strike the Earth? this there was one remarkable quality in the Fog or Smoke of 1783, it appeared to Another remarkable Fog in 1831, which excited the public mind in all quarters of effect it would produce, is that it might inated from some uncommon cause; now the next question is, to what causes shall we as matter into our atmosphere which Now let us look at the facts. It must be acknowledged by all that these Fogs orig ous. (Note 5.) attribute the fogs of 1783 and 1831. Some have supposed that they were caused by These explanations are very unsatisfactory. If the Fogs were actually, produced by bodies which have been for centuries a terror and dread to mankind generally. We must wait patiently until PERIHELION |