suppositions we obtain from (21) for the meridian 93° the excos b' sin (8+8') pression sin -+ sin pression + and for any other meridian the ex cos b sin (+81) sin-"+18 sin-*+igi" Now for the same latitude is less for 93°, or thereabouts, than for any other meridian, while d' is the same; and therefore south of the point of maximum on 93°, sin (8+8') or of latitude 50°, the numerator of the fraction -+ sin is greater and denominator less, and therefore the fraction itself greater upon this meridian than any other. Again near this meridian b' for 10° south of 50° is less than 20; and some 15° from it the value of b exceeds this amount. The maximum value of (21) for the meridian of 93° is then greater than its value at the same latitude (50°) on any other meridian, or at any lower latitude. We have also before made out that it is greater than the value on another meridian at the point for which 8+8=90°, or any where to the north of this. It only remains then to show that it is greater than any value between this point and 50°. The ex1 pression for the maximum value in question is sin" sin-"+18/ · and that for the value of (21) between the two points just mencos b sin (+81) tioned is sin +18 sin We have already seen that the de nominator of the first fraction is less than that of the second for each of the two points in question. As the denominator of the second gradually decreases from one point to the other, it follows therefore that it is continually greater than that of the first. Hence between these points the second fraction is less than the first. We conclude therefore that the pole of maximum intensity is situated about in latitude 50°, and longitude 93° to 100°. In what precedes, it has been supposed that expression (21) is rigorously true, but in its investigation no account was taken of the motion of the fictitious cold poles. The error from this cause is trifling, except at considerable distances from the cold poles, and cannot vitiate the result just obtained. Our conclusions with respect to the situations of the points of greatest intensity on distant meridians, may not be strictly correct, from this cause. A more elaborate discussion will be necessary to fix the precise value of n for all parts of the earth. It is not improbable that it may be necessary in expression (21) to assign different values to n for different quarters of the earth, varying from 2 to 1. In fact (21) cannot be made to extend to the cold poles except by supposing n there equal to unity. If we consult a chart of isothermal lines, we find the greatest variation of temperature to be set down for about longitude 90° W., and latitude 45°; and that this occurs at a lower latitude than the maximum variations on other meridians; which nearly accords with the theoretical conclusion obtained above. It is to be observed that all the considerable changes of total intensity in high latitudes, are almost entirely attributable to changes in the vertical intensity. In latitude 50° on this continent, the entire suppression of the horizontal intensity would not diminish the total intensity more than 0.07. On consulting Mahlmann's table of temperatures, it may be seen that the change in the variation of temperature in the direction towards the cold pole, as we go north beyond 40°, is much less than from the equator to 40°; and that the changes of temperature in this direction accord very well with the changes of magnetic intensity in high latitudes upon this continent, as exhibited by Sabine's chart. GENERAL RESULTS. The following points appear to have been conclusively established by the foregoing discussion. 1. All the magnetic elements of any place on the earth, may be deduced from the thermal elements of the same; and all the great features of the distribution of the earth's magnetism may be theoretically derived from certain prominent features in the distribution of its heat. 2. Of the magnetic elements, the horizontal intensity is nearly proportional to the mean temperature as measured by a Fahrenheit's thermometer; the vertical intensity is nearly proportional to the difference between the mean temperatures at two points situated at equal distances north and south of the place, in a direction perpendicular to the isogeothermal line; and, in general, the direction of the needle is nearly at right angles to the isogeothermal line, while the precise course of the inflected line to which it is perpendicular may be deduced from Brewster's formula for the temperature, by differentiating and putting the differential equal to zero. 3. As a consequence, the laws of the terrestrial distribution of the physical principles of magnetism and heat must be the same, or nearly the same; and these principles themselves must have towards one another the most intimate physical relations. 4. The principle of terrestrial magnetism, in so far as the phenomena of the magnetic needle are concerned, must be confined to the earth's surface, or to a comparatively thin stratum of the mass of the earth. 5. The mechanical theory of terrestrial magnetism which has been under discussion, must be true in all its essential features. 6. We may derive the magnetic elements by very simple formula, and with an accuracy equal to that of Gauss's formulæ, from a very small number of magnetic data determined by observation, and the mean annual temperature of the place. The great importance of these results cannot well be questioned. Whether or not they be regarded as supporting the theoretical views of the physical nature of magnetism which have been briefly alluded to, they cannot fail, it would seem, to throw some additional light upon this hitherto mysterious subject. They also link together in the closest bonds of union, the two sciences of Terrestrial Magnetism and Meteorology, and confer a new value upon the observations made in each of these two great departments of science. ART. XIX.-Notice of Dr. Mantell's Isle of Wight.* WHILE mentioning this new work of Dr. Mantell, we can respond to the sentiment of Scott † In earlier years we visited the "beautiful island," and found in it the most remarkable features -topographical, geological, picturesque and agricultural: while in connexion with the vicinal coast of Dorset, Hampshire and Sussex, it is full of interesting historical associations. Upon this coast the Romans first made their landing-the Saxons, Danes, and Normans followed, and it is easy to understand that the geological events recorded in the strata, and the historical events that have clustered on the surface, may combine to impart to this region peculiar attractions. It lies on the narrow channel which divides England from France, and in a climate which is regarded as the mildest in Great Britain. This region is rich in the fossil treasures of the tertiary, the chalk, and the Wealden; and Dr. Mantell has done much to explore and make known the geology of the S.E. of England. The science is greatly indebted to him for his very able works on local geology, as well as for his comprehensive Survey of the Wonders of Geology, and of the Medals of the Creation. The present work is a large 12mo., of the size of the Medals; it contains 430 pages, and is illustrated by 20 plates, including a geological map of the Isle of Wight, and 36 lignographs or Geological Excursions round the Isle of Wight and along the adjacent coasts of Dorsetshire, illustrative of the most interesting geological phenomena and organic remains. By GIDEON ALGERNON MANTELL, Esq., LL.D., F.R.S., Author of "The Medals of the Creation," "Thoughts on Animalcules," &c. "That beautiful island, which he who once sees never forgets, through whatever part of the wide world his future patli may lead him."-Sir Walter Scott. wood-cuts. It is got up in the same elegant style of paper and typography as the Medals, and is intended as a companion both to the tourist and the geologist. The Isle of Wight, long a favorite resort of travellers and invalids, is now for a part of the year, the residence of the royal family, and of course presents new attractions to a loyal people. The work of Dr. Mantell is therefore in harmony with the era, and as we understand, meets a wide demand. Although the Isle of Wight is visited by the general traveller with particular reference to its picturesque scenery, it possesses, in the language of the author, still stronger claims to the attention of the natural philosopher, for, the strata of which it is composed present phenomena of the highest interest, which elucidate some of the most important pages in the earth's physical history. Notwithstanding the publications of Sir H. Englefield, Mr. Thomas Webster, and other able observers, and of the models of Captain Ibbetson, it is affirmed by the author that the majority of the inhabitants, and that thousands of intelligent strangers who annually traverse the island, pass unconsciously over a country "rich in the spoils of nature and teeming with objects of the highest interest to the instructed observer." This deficiency of knowledge is supplied in the work before us, and its usefulness is not confined to the travellers in this small island, since the phenomena here recorded and the fossils here observed and described, and the conclusions drawn, have an important bearing upon the geology of similar regions found in other countries. It is difficult to give an analysis of a work whose leading topics are necessarily local. Some principal features may however be presented with due prominence. The chalk formation is ably illustrated in this the author is peculiarly at home, as he was born and lived most of his life among the chalk downs and cliffs, and is familiar both with the upper and the lower chalk, forming together with the associated strata, masses of one thousand feet in thickness in the deepest beds. In the instructive summary of the author : The character of the cretaceous system is that of an ocean-bed, formed in a vast basin by successive accumulations of sedimentary detritus, transported by currents, and thrown down in the tranquil depth of the sea; arenaceous and argillaceous deposits prevailed in the lower, and cretaceous in the upper division of the series; periodical intrusions of heated fluids charged with silex having taken place at uncertain intervals. The fossils prove that the ocean swarmed with innumerable beings of the usual orders of vertebrate and invertebrate marine organisms, belonging for the most part to species and genera now unknown; and in the chalk are seen, for the last time, that numerous tribe of Cephalopoda, the ammonites, of which, so far as our knowl edge at present extends, not a single species is known either in the tertiary strata or in more recent deposits: with the chalk the whole race of the ammonites disappeared. With respect to the vegetable kingdom of the cretaceous period, the presence of numerous marine fuci attests the nature of the marine flora; and the fragments of drifted coniferous wood, fir-cones, stems and leaves, which are found in the flint and chalk in some localities, prove that the dry land was clothed with pine forests and cycadeous plants. The occasional discovery of bones and teeth of reptiles, shows that the islands and continents were tenanted by oviparous quadrupeds. Of birds and mammalia not a vestige has been discovered. The Wealden Formation, so ably investigated by Dr. Mantell and other English geologists on the opposite coast of England, has been fully made out in the Isle of Wight, and we may therefore presume that it is continuous beneath the English channel. The clays and fresh-water limestones and beds of lignite of the Wealden, are found at Sandown and Compton bays in this island, and the limestones contain both bivalves and univalves. Petrified trees are found in great abundance at Brook Point, sometimes retaining the ligneous structure, at other times in the form of a coally black matter or filled with pyrites. The trees are all lying postrate and appear to have been accumulated under water like the rafts of the Mississippi. Some of the trees lying along the shore are three feet in diameter, and appear to have been covered when in a state of maturity. Two stems were traced to a length of twenty feet, and indicated trees from forty feet to fifty feet long while living. The annual and annular lines of growth are often very distinct, and bear a close resemblance to the Auracaria or Norfolk Island pine. Thirty or forty have been traced on a single stem; they are all of unequal thickness, thus indicating the variation of the seasons in heat and moisture. The wood was mineralized by calcareous and not by siliceous matter, and the bark was generally turned into lignite. "In the clays, for several hundred yards, both to the east and west of Brook Point, bones of the Wealden reptiles are numerous; with these are associated large mussel shells and lignite." Among the fossil reptiles the bones of the colossal Iguanodon are conspicuous, an animal whose former existence was first proved by Dr. Mantell. These bones are washed out by the sea and strewed along the shore, subject to the destroying action of the waves, and are in general water-worn masses of bone indicating the enormous magnitude of the animals to which they belonged. The bones are generally impregnated with ironoften garnished with brilliant crystals of pyrites, and the medullary cavities filled with calcareous spar. The bones collected within the last few years in Sandown, Brixton, Brook, and |