+211} ; —41, +10h; -16h, and for the temperate zone, +20; 3; +91; — 17", astronomical time reckoned from noon. 2. In the temperate zone, the epochs of the maximum of the morning, and the minimum of the evening, are nearer, by 1 or 2h, the sun's passage of the meridian in winter than in summer; but the type of the summer resembles more the type between the tropics. Observations on the morning minimum are still much wanted. 3. In the torrid zone, the times of maxima and minima are the same at the level of the sea, and on the table lands 1300 or 1400 toises high. We are assured that this isochronism does not show itself in some parts of the temperate zone, and that, at the summit of the great St Bernard, the barometer falls at the hours when it rises at Geneva. 4. The variations everywhere become slower near the concave and convex summits of the curve which represents them; and in some places the mercury seems to remain stationary, during a time varying from 15' to 2h. 5. In general, under the torrid zone, between the equator and the parallels of 15° north and south, the strongest winds, storms and earthquakes, and the most sudden changes of temperature and humidity, neither interrupt nor modify the regularity of the horary variations. Whereas, in some parts of equatorial Asia, as in India, where the monsoons blow with violence, the season of the rains masks entirely the type of the horary variations; and, at the same time, when these variations are insensible in the interior of the continent, on the coasts, and in straits, they show themselves, without any alteration, in the open sea, under the same parallels. 6. Between the tropics, a day and a night are sufficient to determine the times of maxima and minima, and the duration of the small atmospherical tides. In the temperate zone, in 44° and 48° of lat., the phenomena show themselves in all seasons with much distinctness, in the means of from fifteen to twenty days observations. 7. The unequal extent of the diurnal variations produces, in the torrid zone, at the same hours, in different months, differences of barometrical height, more or less considerable. The extent of the oscillations decreases, in proportion as the latitude, and the annual deviations due to accidental perturbations, increase. In the maxima of the evening, the mercury is generally a little lower than in the maxima of the morning. If we confine ourselves to observations that are precise, and sufficiently numerous to give means worthy of confidence, we find, that the extent of the oscillations under the torrid zone, between the equator and the parallel of 10o, in the tide of 9h A. M. to 4h P. M., is, in the plains, from 2.6mm to 3.3mm; on the plain of Bogota 1365 toises high, 2.3mm; towards the extremity of the southern torrid zone, in the plains 2 millimetres. In the whole year, the diurnal variations vary at Bogota from 0.63mm to 3.64mm, and the means of the monthly oscillations vary from 1.5mm to 2.7mm. The extent of the oscillations in the morning tides from 9h to 4h, and in the evening from 4h to 11", are generally, under the tropics, in the ratio of 5: 4 or 5: 3. The mean barometric heights of the days vary between 0° and 10° of latitude, in the plains, 3. 8mm, and on the plain of Bogota 3 millimetres; a difference of level, therefore, of 1600 toises, has a very slight influence on the mean of the daily oscillations, and on the extremes of these oscillations. The mean heights at noon, between the tropics, are constantly some tenths of a millimetre higher than the general mean of the day, deduced from the maximum of 9h A. M., and the minimum of 4h P. M. In advancing from the equator to the poles, we find the differences of the barometrical heights at 9h A. M. and 4h P. M., between 0° and 20° of lat., from 2.5mm to 3.0mm; between 28° and 30° of lat. to 1.5mm; in 43° and 45° of lat. to 1.0mm; in 48°, 49° 0.8mm, and in 55° of lat. to 0.2mm. 8. The barometrical means of the months differ from each other under the tropics from 1.2mm to 1.5mm; at Havannah, Macao, and Rio Janeiro, near the tropics of Cancer and Capricorn, from 7 to 8 millimetres, as in the temperate zone. The extreme deviations of the year are, at the same hours, near the equator, from 4 to 4 millimetres. They rise sometimes at the extremity of the equinoctial zone, near the tropic of Capricorn, to 21mm, and near the tropic of Cancer to 25 and 30 millimetres. In the temperate regions of Europe, the limits of the extreme monthly oscillations are, in the ascending motion, one-half nearer each other than under the tropic of Cancer. In the limits of the ascending oscillations, this difference between the two zones is much less sensible. The interruption of the horary oscillations presents, near the tropic of Cancer (in the Gulf of Mexico) a prognostic of the proximity of tempests, of their force and their duration. The monthly means of the barometric heights diminish regularly, from July to December and January, on the plain of Bogota, and even in the southern hemisphere, on the coast of Rio Janeiro. At the extremity of the northern equinoctial zone, the return of the north winds raises the means of December and January above those of July and August. 9. Under the tropics, as in the temperate zone, in comparing the extreme deviations of the barometer, month by month, we find the limits of the ascending oscillations two or three times nearer than the limits of the descending oscillations. 10. The observations hitherto collected do not indicate any sensible influence of the moon on the oscillations of the atmosphere. These oscillations appear to be owing to the sun, who acts not by his mass, viz. by attraction, but as a star, radiating heat. If, in modifying the temperature, the Solar rays produce periodical changes in the atmosphere, it remains to be explained why the two barometric maxima coincide almost with the warmest and the coldest epochs of the day and the night. The following table will afford a general view of all the horary results: TEMPERATE ZONE. TROPIC. NORTH AND SOUTH TORRID ZONES. EQUATOR. 300 M. Humboldt on the Horary Variations of the Barometer. General View of the Horary Variations in different parts of the Globe, and at Different Heights above the Sea. 2.29 4+I1 mm 2.55 3+11 3.40 2.44 4 +10 +11 3+9 4 +10 5 +91 3+ 10 5 +10 + 101/ Equatorial America, La GuayraN.lat. 10°36′. Plain of Mysore N. lat. Macao, N. lat. 22° 12′. +10 1.75 Cairo, N. lat. 30° 3'. +10 Coutelle. 5 + 10 Win. + 10 4559 4 +11 1.10 +7 Win. 1.00 + 10 Ramond. Sum. + 8 Win. 0.94 Toulouse,N. lat. 43° 34', five years. Chamberry, N. lat. 45° Clermont Ferrand, N. nine years. Konigsberg, N. lat. 54° 52', eight years. ART. XXII.-Notice regarding Professor Mitscherlich's Observations on the Dimorphism of Hydrous Sulphate of Zinc, and Hydrous Sulphate of Magnesia. By WILLIAM HAIDINGER, Esq. F. R. S. E. Communicated by the Author. DURING my stay at Freiberg, I had prepared solutions of sulphate of zinc, and of sulphate of magnesia, to examine and measure crystals newly obtained of the hydrous salts. When the solution of the sulphate of zinc was quite concentrated, and the temperature of the stove rather high, on which I had placed the solutions for producing a slow diminution of heat, I likewise obtained crystals. These crystals, however, bore no resemblance to the ordinary ones produced at lower temperatures, but their forms belonged to the hemi-prismatic system, somewhat like borax, and their degrees of transparency were very low. The sulphate of magnesia, which, on account of the isomorphism of magnium and zinc, I examined under the same circumstances, gave the same result. These facts, isolated as they were, I mentioned to Professor Mitscherlich while at Edinburgh, in 1824. He found that sulphate of nickel, whose second form, a pyramidal one, had been described before, did not yield a hemi-prismatic species, when exposed in the same manner to a higher temperature. Some time afterwards, when he was examining the changes produ`ced by heat in the double refraction of crystallized bodies, he observed that it remained unaltered in the hydrous sulphate of magnesia, till at once the whole crystal, which was heated in oil, became opaque. On being broken, it showed the structure of a pseudomorphous crystal, consisting of a number of individuals, beginning at the surface, and meeting in the inside of the original crystal. Professor Mitscherlich repeated the experiment under various modifications, from which it appeared, that this change always ensued at a temperature of about 42° R. (126° Fahr.) both in the sulphate of magnesia, and the sulphate of zinc. When the crystal is exposed in a glass tube to the heat of the spirit-lamp, its decomposition takes place without the loss of water, except, perhaps, what has been me |