124 



* KNOWLEDGE ♦ 



[Feb. 13, 1885. 



To these add Croll's special calculation for the maximum 

 eccentricity between 851,000 n.c. and 849,000 B.C. 



Ycirs B.C. EccMitricity. 



851,000 007454 



850,000 00716G1 



8-i',),500 007-100 



840,000 007450 



The above table is the result of mathematical computa- 

 tions not open to doubt or question. Any one who 

 examines it will find a remarkable discrepancy between 

 the actual movements of the perihelion and those in the 

 books. These last resulted from a misapprehension of 

 certain results presented by Lagrange and Laplace (inde- 

 pendently), indicating a certain progiesbive movement of 

 the perihelion, which can be emjiloyed in all formula 

 extending over the range of time covered by the history of 

 astronomy. ITsed as they have been, as if they applied 

 to hundreds of thousands or millions of year.-i, they lead to 

 entirely erroneous conclusions. 



In passing, also, I may note another common error. 

 Many imagine that the path of the earth is obviously 

 elliptic as well as obviously eccentric, and suppose that it 

 is only the carelessness of astronomical draughtsmen which 

 has caused the earth's path to be always represented as 

 circular. But, as a matter of fact, while the eccentricity is 

 obvious enough on the scale of the usual astronomical 

 diagrams, the ellipticity is too small to show. Even at the 

 time, 850,000 years ago, when the earth's orbit had the 

 greatest eccentricity it has had for the last two millions of 

 years, and very nearly the greatest it can have, the shajie 

 of the path was appreciably circular. Putting the mean 

 distance at 13, and the eccentricity (regarded here as a 

 distance) at 1, the departure from the circular form (that 

 is the greatest distance by which the earth's elliptic path 

 fell within the inclosing circle) would be represented by no 

 more than the diflerence between the square root of 109 

 (i.e., 13) and the square root of 168 (about 12 90). The 

 numbers 325 and 321 represent the actual proportion 

 between the greatest and shortest axes of the earth's orbit. 

 So that if a circle is drawn with a radius of 3] in. it 

 nowhere de])arts more than the 100th part of an inch 

 from the ellipse which would represent with perfect 

 accuracy the orbit of the earth 850,000 years ago, when it 

 was so much more eccentric than it is now. (See figure at 

 page 89, No. 170.) 



"Tau Sigma's" theory was associated with somewhat 

 daring ideas as to southern glaciers far vaster than any 

 now existing, and he advanced a new interpretation of the 

 glacial epoch. Lyell's interpretation, based on Croll's 

 calculations, seems quite sufficient to meet all the require- 

 ments of the case. But there can be no doubt that the 

 great Antarctic Ocean region is a terrestrial feature well 

 worth careful consideration. If Adhemar's theory, that the 

 northern and southern hemispheres alternate as the chief 

 oceanic regions, had not been accompanied with so much 

 that is unsound, it probably would have been regarded as 

 well worthy of consideration in this connection. There can 

 be no doubt whatever, it seems to me, that the present 

 arrangement is the result of a di.-,turbance which has 

 caused the centre of gravity of the earth's solid mass— or, 

 to be more precise, of that portion of her mass which is 

 bounded by a solid surface— to lie measurably south of the 

 centre of figure. 



It is certain that the oceanic surface in the soiithern 

 hemisphere passes outside the mean surface which the ocean 

 woiild have if the attractions of the solid globe were more 

 uniformly directed towards (not precisely to) the centre of 

 figure. The sufficient and convincing proof of this is found 

 in the forms of terrestrial promontories, which show that 



the general surface of the solid crust is not concentric with 

 the surface of the watery envelope. But there is another 

 proof scarcely less convincing which I pointed out seven- 

 teen years since — I mean the low barometer of the southern 

 hemisphere. If the sea level rises above its natural mean 

 level in the southern hemisphere the sea surface there would 

 present the constant characteristic of high levels — a low 

 barometer. Now this is precisely what has long been 

 recognised, and had long been regarded as mysterious, 

 though Maury supposed he had found the interpretation. 

 The seas are drawn southward becau.se the earth's centre 

 of gravity is south of the centre of figure. The water 

 thus drawn southward stands m effect as a raised surface 

 above the mean level. The air is also drawn southward ; 

 but the centre of gravity for the air is the centre of gravity 

 of tlie entire mass formed by the solid earth and the waters 

 of the sea. The mean density of the solid earth being 

 nearly six times that of water, it follows necessarily that 

 the centre towards which the air is drawn, though south of 

 the centre of figure of the solid earth, is north of the centre 

 of figure of that incomplete sphere of whose surface the 

 surface of the ocean is part. Hence, estimated for the sea 

 level, atmospheric pressure is higher in the northern than 

 in the southern hemisphere. Whether with a change in 

 the position of the perihelion the water will be drawn 

 northward, and these relations reversed, remains to he 

 seen — a few thousands of years hence. Let us be patient. 



THE WORKSHOP AT HOME. 



By a Working Man. 



AVERY handy, in fact almost indispensable, thing in 

 every house is a step-ladder, or, as it is nearly always 

 called, "a pair of steps." After the rather more com- 

 plicated work which we have lately been about, the reader 

 will find it a comparatively easy job to make one. Fig. 37 



Fig. 37. 



gives a side-view of our steps when completed. For the 

 sides, s, s, the steps, st, st, and the top, t, we shall use deal 

 1 inch thick in the rough ; this will plane up to the proper 

 size. We will make the sides 5 feet 6 inches long, a very 

 handy size, and they shall be 4 inches wide. This will 

 enable us to get two out of a 9-inch plank. The steps, st, si, 

 are also cut out of 1-inch stuff" (I mean 1-inch before it is 

 planed up), and are 5 inches wide, so as to project an inch 

 in front of the sides. Counting the top, f, there will be seven 

 steps 9 inches apart. They are let into the sides by grooves, 



