556 



NATURE 



{Sept, 19, 1878 



and indeed a marine or rather a brackish plant, closely related 

 to the species of > the present genus Chorda^ Stack. This frag- 

 ment seems to have been mixed in the tide pools with fi-esh 

 water or land plants growing there. For another thick specimen 

 of the same locality and compound bears a profusion of marine 

 niollusks, and has only branches of this as yet undescribed 

 marine species, Calamophycus septus. 



Habitat lower heldeberg sandstone, Michigan, discovered and 

 communicated by Dr. Carl Rominger (State Geologist). 



On comparing my Manx specimen, which was found on the 

 surface in a field at Laxey with that figured and described by 

 Prof. Lesquereux, it agrees with the latter in every respect, 

 except that striae and scales are not observable on the stem. The 

 stem is thick, dichotomous ; divisions variable in distance, the 

 terminal ones short, pointed nearly equal in size and length, 

 surface nearly smooth. The branches in the lower part are thick 

 comparatively to their length. The surface of the stem appears 

 to be smooth and affords no evidence of strise or scales. 



The woodcut on the preceding page represents the specimen 

 a little over the natural size. 



The stone in which the plant is embedded is a fine-grainetl 

 grit of a grey colour, and the specimen itself is of a yellow tint 

 as if coloured by oxide of iron ; it nins nearly at right angles to 

 the bedding of the stone, and appears as if standing in the same 

 position as it had grown. The stone is a rolled one but it is 

 evidently from the Manx schists found in the vicinity. These, 

 according to Profs. Harkness and Nicholson, are of the age of 

 the Skiddaw slates, but the rock in which the fossil occiurs may 

 be of older date, as some of the lower portions of the series have 

 not yet been clearly determined ; so here we have evidence of a 

 plant in the lowest part of the silurian formation, or even lower. 

 By diligent search the rock in which the specimen occurs may 

 probably be found in sitA in the upper part of the Laxey valley. 

 The great resemblance, if not the identity, of the Manx with the 

 American specimen is very remarkable, and shows the similarity 

 of conditions then prevailing in distant parts of the globe. The 

 specimen might have been called Psilophytiim cornutum, if any 

 marking on the surface of the stem had been observed, but as 

 these appear to be absent it is proposed to call it Psilophytiim 

 moneiise. As to the nature of the water in which it grew there 

 is no evidence from organic remains, but its characters resemble 

 those of a fucoid more than a land plant. 



THE FIGURE AND SIZE OF THE EARTH^ 



npHE portion of the earth's surface bounded by the horizon 

 ■*• which one is able to take in at one view, is but seldom a 

 regular plane ; more generally heights and depressions, mountains 

 and valleys, alternate with each other so irregularly, that at first 

 nothing seems farther from reality than the idea of a regular 

 form of the earth's surface. But the more our point of view 

 overtops the mountains which lie within the horizon, the further 

 obviously will our range of view extend, and all the mountains 

 and valleys which give so irregular a form to the horizon of the 

 plain will, under this condition, become imperceptible and un- 

 important. Indeed, one can easily conceive that if the eye were 

 able to comprehend at one time a much greater portion of the 

 surface, the irregularities of the plain caused by the mountains 

 and valleys would appear exceedingly small in comparison with 

 the extent of surface. But such considerations must also have 

 occurred to the ancients ; for the earliest conception among 

 the Greeks of the form of the earth's surface was that of a flat 

 disc surrounded by the river Okeanos, into which the sun plunged 

 nightly. The first advance was made by Thales, who said the 

 earth must have a point of support, and imagined it was 

 borne by the water. Anaximenes supposed that a strong dense 

 atmosphere supported the earth. Quite another idea prevailed 

 in India, where the earth was believed to be borne on the back 

 of an elephant. More correct views of the figure of the earth 

 prevailed at an 'earlier period in other parts of the East, in 

 Egypt and a part of Asia. The Egyptians and Chaldeans 

 taught at the earliest period the spherical form of the earth, and 

 Pythagoras appears to have adopted this idea from them. 



This difference of conception need not, however, be wondered 

 at when we remember that the Greeks seldom undertook long 

 journeys, and knew of the lands outside Greece only from 

 fabulous narratives. It was otherwise with the people of the 

 East, who, through their frequent and extensive travels, learned 

 at an early period to know the positions of the stars as guides, 

 ' From a series of papers in Die Natur, by Karl Maria Friederici. 



and attained to a more correct conception of the size and form 

 of the earth. The Chaldreans already knew the circumference of 

 the earth so nearly that they said a good walker would take three 

 years to v/alk round it. 



Eudoxus was the first in Greece to recognise a symmetrical 

 curvature of the earth's surface. He had noticed on long journeys 

 that stars which at their greatest height (culmination) stood near 

 the horizon gradually diminished in altitude, and finally disap- 

 peared ; but on his return to those regions they again gradually 

 became visible and assumed their previous altitudes. The cir- 

 cumstance that these altitudes of the stars changed regularly in 

 proportion to the length of way travelled, led him to the con- 

 clusion of a regular curvature of the earth's surface. This 

 conclusion being accepted, a simple method was indicated for 

 measuring the circumference of the terrestrial sphere. For sup- 

 pose a star reaches at a place, A, at its maximum a height 

 of seven degrees above the horizon, if the observer move 

 to another place, B^ lying to the north, but in the same geo- 

 graphical longitude as A, and measure again the highest altitude 

 of the same star, say six degrees ; then the distance of the place 

 A from B is equal to the 360th part of the whole circumference 

 of the earth. Let the distance between A and B be now 

 measured, and it will be found to be sixty-nine English 

 miles ; thus the entire circumference of the earth would be 

 69 X 360 = about 25,000 miles. 



Aristotle inferred, from physical and especially hydrostatic con- 

 siderations, that the earth was spherical, since, he said, the water, 

 which formed the larger part of the upper stratum of the earth, 

 sought, by virtue of its weight and the mobility of its molecules, 

 to approach as near as possible to the centre of the earth, 

 it sought to assume the lowest position, and could never be in 

 equilibrium until all parts of its surface were equidistant from 

 the centre of the earth, i. e. , formed a globular surface. This 

 inference, near as it comes to the truth, was yet in, Aristotle's 

 time only an unproved hypothesis ; the existence of a centre 

 exerting attraction in all directions was first recognised as pro- 

 bable at a much later period, Newton being the first to publish 

 the conception. 



The theory according to which the earth is a spherical body, 

 was more and more generally accepted, and was put beyond 

 doubt when the first circumnavigation by the Portuguese 

 Magellan (1519) became known, an example followed, at short 

 intervals, by almost all European nations. Thus the idea so 

 generally accepted at a very early period that the figure of the 

 earth must be spherical, was again revived about the end of the 

 seventeenth century. The desire to ascertain, according to the 

 above-described methods the circumference of this circle was 

 also cherished by the ancients, and we have accounts of measure- 

 ments taken for this purpose in the earliest times, of the most 

 important of which we give some account. 



The first determination known to us of the size of the earth was 

 made by Eratosthenes in Alexandria in the third century before 

 Christ. He observed at the solstice (the time of its greatest 

 northern declination) in Alexandria, the greatest altitude of the 

 sun above the horizon, and it was known that at that time the 

 sun stood when at its greatest altitude, in the zenith at Syene (from 

 which we may conclude that it could be seen in a deep well). 

 Now since the altitude of the sun above the horizon is always 

 equal to 90° minus its distance from the zenith, he thus re- 

 quired only to subtract the measured height from 90", and 

 thus found the distance from the zenith to be the fiftieth 

 part of the whole circumference, or 7° 12'. According to 

 this process the distance of the two places.was regarded as a 

 fiftieth part of the earth's circumference ; and as that distance, 

 according to the accounts of travellers, was 5,000 stadia, the 

 whole circumference of the earth was equal to 250,000 stadia. 

 Eratosthenes altered the result to 252,000 stadia, taking for 

 the length of a degree, 700 stadia. Without considering the great 

 inaccuracy of his altitude measurements, there are yet too many 

 other formidable sources of error in this estimate of the earth's 

 circumference, to allow it any claim to much accuracy. First 

 there was the taking for granted that both places lay on the 

 same meridian, which was not the case, since Syene lay three 

 degrees east from Alexandria ; and second, the distance of the 

 two places reckoned at 5,000 stadia was too great. 



A second investigation was made by Posidonius in the first 

 century before Christ, but his result was still more erroneous than 

 that of Eratosthenes. He observed the height of one of the 

 brightest stars (Canopus in Argo) above the horizon. It reaches, 

 at the time of its culmination at Alexandria, an altitude equal to 

 the forty- eighth part of the circumference, while in Rhodes it was 



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