Dec. 4. 1879] 



NATURE 



1C5 



CHALLIS'S "PRACTICAL ASTRONOMY" 

 Lectures on Practical Astronomy and Astronomical In- 

 struments. By the Rev. James Challis, M.A., F.R.S., 

 &c. (Cambridge : Deighton, Bell, and Co. ; London : 

 George Bell and Sons, 1879.) 



AS LONG since as the year 1S43 the Plumian Profes- 

 sor of Astronomy and Experimental Philosophy in 

 the University of Cambridge commenced a course of 

 lectures upon practical astronomy and the use of astrono- 

 mical instruments, the proximity of an observatory pro- 

 vided with instruments of first-rate quality appearing to 

 him to give facilities for acquiring a knowledge of the 

 practical branch of the science which ought to be taken 

 advantage of. The syllabus of these lectures has been 

 utilised as the groundwork of the present volume, but 

 the progress of astronomy has necessitated the introduc- 

 tion of various modifications in the original plan and 

 contents. 



By far the larger portion of the volume is devoted to 

 the description and use of instruments found in the larger 

 fixed observatories, the illustrations being usually appli- 

 cable to instruments in the Cambridge Observatory, and 

 the work being therefore more especially adapted to the 

 use of students in that university, who have frequent 

 access to the observatory. Notwithstanding this circum- 

 stance, however, the treatise will be found to convey 

 much information on practical matters and details, which 

 is not of so limited an application. 



The fixed instruments which are treated of at length 

 are the transit instrument, the mural-circle, the transit- 

 circle, incorporating in its employment the purposes of 

 the two former instruments and the equatorial. The 

 transit-circle may now be said to have wholly superseded 

 the mural-circle and transit-instrument in most of the 

 principal observatories, but the explanations of them 

 given by Prof. Challis being applicable to the reduction 

 of meridional observations generally, are also applicable 

 in great measure to the transit-circle. The latest obser- 

 vations with a mural-circle which we remember to have 

 seen were made at the Naval Observatory, Washington, 

 and the instrument still figures in the last description of 

 that establishment. Prof. Challis enters fully into such 

 details as the mechanical adjustments of a transit instru- 

 ment and the correction of errors by calculation, the 

 construction and application of the collimating eye-piece, 

 the azimuth-error obtained by astronomical observations, 

 and correction for error of level, also upon a method of 

 correcting the errors of a transit-instrument for deviation 

 of the pivots from a cylindrical form, a defect which, if 

 wc are not mistaken, considerably exercised not only Prof. 

 Challis, but his successor Prof. Adams, as exemplified in 

 the Cambridge instrument. The computation of apparent 

 and mean right ascensions follows, with reference to 

 recent corrections of the more important astronomical 

 constants entering into such work. Similarly the author 

 describes the method of observing with the mural- 

 circle, and the calculation of the mean polar distances of 

 stars and the geocentric polar distances of the sun, moon, 

 and planets with numerical examples. The equatorials of 

 the Cambridge Observatory and their appliances are 

 particularly described, and the adjustments of this form 

 of astronomical instrument and the method of deducing 



the right ascension and polar distance of a celestial 

 object from the observations. Reference is also made to 

 the "counterpoise mounting" of an equatorial, and the 

 arrangements, advantages, and disadvantages of this form 

 of mounting. 



Other fixed instruments of which some account is 

 given are the altitude and azimuth instrument, with 

 special reference to that at the Royal Observatory, Green- 

 wich, the zenith sector, Airy's reflex zenith tube, and the 

 transit in the prime vertical. There are also articles upon 

 transportable instruments, as the theodolite and sextant, 

 on the chronographical method of registering transit- 

 observations, the different methods of determining terres- 

 trial longitudes, the solar paiallax, and miscellaneous 

 additional subjects of less importance. 



The volume, as we have already stated, and, indeed, as 

 is hinted by Prof. Challis, may appear to be more especially 

 adapted to students in the university who desire an 

 acquaintance with the principles of astronomical practice, 

 and in his preface the author urges the advantages of 

 giving attention to a subject of this kind, "as a mental 

 exercise of much educational value, inasmuch as it is 

 altogether unlike any process of reasoning by abstract 

 symbols, and may serve as a corrective to the effect of 

 too exclusive an attention to reasoning of that kind." 

 But there are few works of its nature in English astro- 

 nomical literature, a circumstance which is likely to 

 secure for it a wider circle of readers. 



The volume has been published at the expense of the 

 Syndics of the University Press. 



OUR BOOK SHELF 

 The Carboniferous Limestone and Cefn-y-Fedm Sand- 

 stone of the Country between Llanymynech and Minera, 

 North Wales. By George H.Morton,F.G.S.,F.R.G.S.I., 

 Honorary Secretary of the Liverpool Geological Society. 

 (London : David Bogue, 1S79.) 

 In this exc llent monograph, which is now reprinted 

 from the Proceedings of the Geological Society of Liver- 

 pool, Mr. Morton has brought together a vast amount of 

 valuable information concerning the lower carboniferous 

 rocks, as displayed in a district where they have been 

 but little studied. The splendid escarpment of the 

 Eglwyseg rocks, near Llangollen affords a series of mag- 

 nificent sections, the interpretation of which, however, 

 requires considerable patience, care, and caution on the 

 part of the field geologist. Mr. Morton shows that the 

 succession of the lower carboniferous rocks in North 

 Wales presents marked points of difference from that 

 which is observed either in Yorkshire or Derby-hire. He 

 divides these lower carboniferous rocks into two groups — 

 "the carboniferous limestone below and the Cefn-y-Fedw 

 sandstone above ; and in the correlation of these he adopts 

 the views originally put forward by Prof. A. H. Green, 

 and since supported by Mr. D. C. Davies, namely, that 

 the conglomerate and sandstone strata overlying the car- 

 boniferous limestone represent not only the millstone 

 grit, but also the Yorcdale rocks of Prof. Phillips and the 

 Geological Survey. 



Mr. Morton shows that the carboniferous limestone of 

 this district attains a thickness of 1,200 feet, and 

 detailed descriptions of the several members of 

 is made up, with lists of the fossils obtained from each. 

 The overlying arenaceous formation is 723 feet thick, 

 and consists of alternations of sandstone conglomerate 

 and shale; these beds contain marine shells, with some 

 plant-remains. The sections which are given in this 



