August 1, 1884.] 



SCIENCE. 



101 



and more thoroughly cut down, the river's 

 wearing and transporting power being in pro- 

 portion to the great beds of shale, sandstone, 

 and limestone. 



In many localities in the state of New York 

 and elsewhere, there are glens and ravines cut 

 wholly out of the Genesee and Hudson River 

 shales, where there are no alternations of hard 

 and soft strata, as in the Portage. Precipitous 

 hillsides are also of frequent occurrence, al- 

 though the face of the rock soon turns to soil. 

 The reason why the edge of apparently so soft 

 a rock of such fine material withstands the 

 weather, and presents these naked sections for 

 such a length of time in mural banks in ravines, 

 river-courses, and upon the shores of lakes, is 

 on account of its uniformly foliated structure. 

 A very slight examination will serve to show 

 the thin laminae of which the entire rock is 

 composed, like sheets of paper, reminding one 

 of the resisting power of the edge of a book. 

 The hardness of some kinds of coal is also 

 owing to its laminated formation. A precipi- 

 tous wall, whether built by nature or by art, 

 must either be laid with a good cement, or 

 it must be composed of material having a 

 good bed, ' breaking joints ' both inward and 

 laterally. 



A peculiarity of the loess or bluff formation 

 on the Mississippi and Missouri Rivers is, 

 that although it is ver} T fine, soft, and easily 

 excavated with the spade alone, 3-et it presents 

 very steep slopes and precipices resembling 

 those of solid rocks. Unlike all other forma- 

 tions of an earthy nature, it remains unchanged 

 by the atmosphere and the action of frost. 

 Road-cuts and embankments, however steep, 

 stand for years like a wall ; and wells dug in it 

 require to be walled onty to a point above 

 the water-line, while the remainder stands so 

 securely without support, that the spade-marks 

 remain upon it for years, although it is not 

 at all cemented together. In the city of St. 

 Joseph, and all other places where the bluff 

 formation is found, these peculiarities can be 

 easily seen ; and they appear very remarkable 

 to an eastern man, accustomed to the sloping 

 down of banks of sand and clay. The ex- 

 planation of it is, that, as is well known, the 

 bluff is a lacustrine deposit. The material 

 forming it floated in flakes in a quiet, shallow 

 lake. The minute particles, assuming a flat- 

 tened form, however it ma} T have been caused, 

 were very quietly and gently deposited in lay- 

 ers, like little sheets of paper. There was no 

 current, no movement of the particles to form 

 rounded grains of sand, irregularly deposited 

 in accidental disorder. On the contrary, the 



bluff is a well-built piece of miniature natural 

 earth masonry, well bound together : hence 

 there is no rolling tendenc} T in the material, and. 

 when cut down at right angles to the layers, it 

 does not form a slope, like other kinds of earth. 

 Thus, from precipices of rock of the heavier 

 strata to those composed of the smallest, 

 their mechanical structure is of great impor- 

 tance, and the same homely comparison of the 

 ' stretchers and binders ' of an artificial wall 

 applies. 



James Macfaklane. 



Towanda, Penn. 



THE EQUATORIAL COUDE. 



In spite of the loss of light in the two] reflections 

 from its mirrors, — which loss will vary with the 

 condition of the reflecting silver films, but, under 

 the best conditions, should not much, exceed twenty 

 percent, — the equatorial coude of the Paris obser- 

 vatory would seem to be the coming form for nine- 

 tenths of the equatorial work of an observatory. 

 This form of ' elbow equatorial ' has been described 

 of late in so many scientific periodicals, that it is suf- 

 ficient here to say that the polar axis forms a part of 

 the tube, at the upper end of which the observer sits 

 like a microscopist at his desk, and at whose lower 

 end a 45° mirror turns the course of the rays into a 

 tube at right angles to the axis ; and at the outer end 

 of this tube is the objective, with still another 45° 

 mirror outside of it, which turns round the axis of 

 this tube. This gives the motion in declination, and 

 the rotation of the whole round the polar axis gives the 

 motion in right ascension. All the movements, 

 the reading of all the circles, the illumination, and 

 every thing connected with the management and use 

 of the telescope, are directly under the observer's 

 control as he sits at his desk, where there is every 

 facility for attaching spectroscopic, photometric, and 

 micrometric apparatus to the eye-piece end, which 

 keeps its fixed position. Moreover, the observer and 

 all this accessory apparatus can be entirely roof ed in, 

 and the room warmed in cold weather, if desired, and 

 the observer made as comfortable, and the work as 

 convenient, as that in any laboratory, while the 

 whole heavens are at his command. 



There can be no question as to the desirability of 

 this, when compared with the discomfort and exposure 

 in the common observatory dome, and with the dif- 

 ficulty of attaching accessory apparatus to, counter- 

 poising, and using it upon, the moving end of an 

 ordinary equatorial. Still further, the observing- 

 room can be made entirely dark when desired ; and 

 the increased sensitiveness of the retina, under these 

 circumstances, will be a great gain in delicate spectro- 

 scopic and photometric work. Also, in work upon 

 the sun, the possibility of protecting the accessory 

 apparatus entirely from the sun's direct rays, and 

 even of working in the dark if desired, will be a 

 great improvement upon the inconveniences unavoid- 

 able in the common observatorv dome. 



