ing all tended to a more perfect knowledge of the affinities of plants, we are assured have been the effect of progress in the right direction." Without doubt the preponderating tendency of the ablest and most experienced botanists of the present day is, to cancel nominal species, and, taking a more enlarged view of specific characters, to reduce slight or varying modifications to a common type; a point not yet reached by zoologists, though probably it will be hereafter. Dr. Hooker's tendency in this direction is evidently very decided; possibly too much so. But it must be allowed that, while the botanists who multiply species unduly are always those who work upon scanty materials, or whose personal observations are limited to a single district of country; on the other hand, those who have access to the largest collections, or who have themselves botanized over various parts of the world, are for the most part as strongly inclined in the opposite direction. Now Dr. Hooker possesses both these advantages in an eminent degree. Young as he still is, no living botanist has investigated on the spot so many and so widely separated floras, and few like him have had constant access to the largest and best determined herbaria in the world. The principal danger here arises from l'embaras des richesses. It is hardly possible that a vast series of apparently confluent forms should receive the detailed examination which the less privileged botanist may concentrate upon his fewer materials; and much is left to the quick, almost intuitive judgment, which is liable to error, indeed, but in which the true genius of a botanist is generally disclosed. Perhaps there is no equally well known flora which compels the botanist to allow of such wide limits of variation in so large a proportion of its species, as that of New Zealand: at least so it would appear at first sight. Yet this character seems to be exhibited more or less by insular floras generally, in which a considerable number of the peculiar species are apt to be surprisingly polymorphous, as was remarked by Bory de St. Vincent half a century ago. (To be continued.) A. G. ART. XXVII.-On the relations which exist between Friction and Pressure; by M. J. NICKLÈS. IN preceding numbers of this Journal I have proposed for the Mechanical Arts the use of magnetic adhesion, a principle purely physical; and it becomes necessary to ascertain the relations which exist on a horizontal plane 1. Between pressure and the friction of iron sliding on iron. 2. Between the pressure which one of these masses of iron converted into an electro-magnet exerts on the other, and the ef fort to overcome it in order to make this electro-magnet move on its armature. The Treatise on General Mechanics by Morin does not mention the first relation, that of the friction of iron on iron. It is well known however, that in practice the number 0 33 is adopted for it. I have reasons for thinking that the relation is the same in amount between magnetic pressure and the friction of sliding under this new condition. If the experiments which I have made on this subject do not prove that the friction is equal to one-third the pressure, they tend at least to establish an identity of relation between the two pressures and the two frictions. I have had occasion to recognize a like identity between the adhesion or friction of rolling under the influence of added weight alone and the adhesion or friction of rolling under the influence of magnetic attraction.* The electro-magnet employed in the experiment was one of Joule's. The helices consisted of 30 metres of copper wire, 2 mm. in section; each of its polar surfaces were 0.080 m. in length and 0.020 m. in breadth. They are shown in figures 1 and 2, which represent, in elevation and section, the apparatus as well as the different phases in the experiments. The armature consisted of a piece of a railroad rail, e, placed level on a bar of wood, s; the magnet and armature had been carefully cleaned and polished. A cord attached to the axis of the magnet was connected with a system of pulleys, x, x, and at the other extremity was attached See this Journal, vol. xvi, p. 387, Nov., 1853. Phil. Mag., [4], ii, 450. a scale for receiving the necessary weights, to determine the force of the magnetic attraction. This force, once known, the cord attached to the axis of the magnet was detached, and another was applied a little above the poles, at the height of a small pulley, y, fastened to the extremity of the rail: this cord which passed between the arms of the magnet terminated in a small rod of copper which was secured to f, on the arms, where it was held by rivets. A second scale, P, smaller than the preceding, received the weights required for moving the apparatus onward, in the plane of the rail. The experiment was tried with a dry armature, and also with one wet with well water. To the weight carried, given in the table, 4 kilograms should be added, for the electro-magnet, and this is noted in the column of means. Moreover the horizontal sliding observed was a kilogram too great, this representing the effort to be overcome in causing the electromagnet to slide when not magnetized, and this is deducted in the column of means. The results with the dry and wet rail are The circuit being broken, a weight of 232 kil. was added to the magnet, by means of the bar m m, supported on the top of the magnet; the cord directly below carried a pulley to which a scale for the weights was adapted. The pressure was thus exerted normally on the magnet. The magnet itself weighed 4 kilograms; the charge was therefore 236 kil., which represents the pressure exerted on the armature in the preceding experiment. Normal pressure. | Means. Horizontal sliding. Means | Relations. It would have been interesting to have determined the amount of effort required to make the apparatus slide when both magnetized and weighted, to ascertain if the result would be the sum of the two slidings. But an accident which happened prevented the repetition of the experiments which were made in the month of January, 1851. ART. XXVIII.-Abstract of a Meteorologial Journal, for the Year 1853 kept at Marietta, Ohio, in Lat. 39° 25' N., and Long. 4° 28' West of Washington City; by S. P. HILDRETH, M.D. The temperature for the year is 520.74, which is rather above the mean annual heat for this place. No very remarkable extremes of heat or cold have distinguished the year, although the temperature in June and July was rather excessive, rising on the last day of June to 95°, being higher than for several previous years, by two or three degrees-on a number of days it was above 90°. My thermometer had a northern exposure, on the back side of the house, protected from the reflected rays of the sun by trees. At Cincinnati the degrees of heat were considerably higher, rising to 90°; and on a number of days to 95° and 96°. In New York, by the aid of reflected heat from the brick walls and stone pavements, the temperature was more elevated than in any former year; rising to above a hundred, and causing the death of a large number of persons. Heat, alone, even to 110°, is not necessarily fatal; as in the gold diggings of California, and at Mosul, in Assyria, on the river Tigris, where the American Board of Missions have several missionaries, whose health remains good with the thermometer at 1120 or higher. To render it deadly, requires some imprudent act, as drinking cold water, which paralyzes the motion of the heart, by its sudden reduction of temperA benevolent and all-wise creator, has regulated the laws of nations in a way most beneficial to man; so that water from springs and fountains shall be of the same temperature as the mean annual heat of the climate; thus in a measure preventing the injurious effects of cold water on the over-heated human frame in the tropical regions of the earth. ature. The amount of rain and melted snow was 37% 04 inchesbeing nine and a half inches less than in the year 1852; but this is not the lowest in the scale, as it is sometimes reduced to thirtyfour inches. The Ohio river was low all the summer, commencing early in June, as we had no rain at the summer solstice, a period when a rise in the Ohio is always expected, and seldom fails to appear. With the exception of a few feet of elevation in November, the water continued low until the setting in of winter, and closed with ice on the 25th December, when steamboats ceased running. The fall of snow was light, only about five inches near the end of the month. The lowest temperature in December, was six degrees above zero. Winter. The mean of the winter months was 360-90. The coldest day was on January 29, when the mercury fell to zero. In February the lowest was 11° above. Great fluctuations of temperature occur in this month, as in 1818, the thermometer fell on one morning to 20°, and on the next to -22°; which destroyed all the peach trees in range of its influence. In, January 1851, the extreme cold being 23 below zero, killed a large portion of the bearing trees in the valley of the Muskingum; but as it was not repeated the following night, the effect was not so entirely disastrous, as some trees produced fruit in 1852, but have since died. In the latter part of January, the navigation on the Ohio was impeded by floating ice, but the river was not closed during the whole winter. There was but little snow, not more than four or five inches, and at any one time not over two inches in depth. The winter was a very mild one. Spring. The temperature of the spring months was 510-55, which is rather below the mean for this climate. The amount of rain was quite abundant, especially in April, and is very important, as laying the foundation for the grass crop and other cereal productions. was Summer-The temperature of the summer months 720-43, which is higher than any previous one for many years, the mean average being about 70°, or rather less. The heat commenced in June, and one of the hottest days was the thirtieth, rising to 95. July is generally the hottest month, but in 1853 the temperature was three and a half degrees below that of June, being only 710-15, and the latter 74° 60. At Cincinnati the heat was considerably greater than in Marietta, rising on two days to 98°, and on several to 94°. The mean for the month was five degrees above us, or 790-32. It was six degrees above the mean for June in the last ten years. With this greatly increased range of the thermometer there was a less amount of rain than usual, there being quite a drought in June and July, which was doubtless one cause of the heat. It had a favorable effect |