152 



NA TURE 



[June i8 1908 



A ■ FIELD METHOD OF DETERMINING 



LONGITUDES BY OBSERVATIONS OF 



THE MOON. 



A SHORT note appeared in these columns on 

 April 23 (p. 590) with reference to a valuable 

 paper by Mr. E. B. H. Wade, published by the Survey 

 Department of Egypt. The paper, however, is one 

 showing so much originality, and the instrument and 

 method appears to be of such value, that a more 

 extended notice is called for, the more so as the paper 

 in question is by no means generally accessible. 



^Ir. Wade considers the case of field survey 

 operations where at least one point, and preferably 

 more, should be determined independently by a direct 

 astronomical method. Latitudes are easy enough, and 

 longitudes would be if telegraphic connection with an 

 observatory were available. Until wireless time- 

 signals are distributed over the world which might be 

 picked up with portable receiving apparatus, the ob- 

 server has to rely upon his chronometers, which become 

 cumulatively misleading, or upon lunar observations. 

 These latter, Mr. \\'ade points out, may be divided into 

 two classes — those which depend directly upon the 

 rotation of the earth, of which moon culmination ob- 

 servations serve as an example, and those which do 



The telescope t with its reflector f is mounted as an altazimuth, the motion in altitude being given by the handles L. 

 B. the box-sextant, containing the horizon mirror H and index mirror I, is actuated by worm gear by means of the long 

 handles, and so is made to turn about the reflected optic axis of the telescope, i.e. about a line at right angles to the 



axis of the actual telescope. 



not, for instance, lunar distances and occultations, 

 and these have the advantage of being about thirty 

 times less sensitive to errors of time. 



Mr. Wade has set himself to devise a method of 

 making lunar distance observations with an accuracy 

 outside the possible range of the sextant with apparatus 

 that is easily portable. He refers to Talcott's method 

 of determinmg latitude by observations of the equi- 

 distance of the zenith from two stars as compared with 

 the actual measurement of its distance from a particu- 

 lar star. " In quite the same way " he suppresses 

 " the graduated circle of the ordinary sextant and finds 

 the moon's apparent position from the condition that it 

 is equidistant from two or more stars, one of which, of 

 course, must precede and the other follow the moon." 

 Bv suppressing the graduated circle he removes the 

 principal obstacle to the evolution of the sextant. 



In order to arrive at this result, he starts with a good 

 telescope of 2i inches aperture and magnifying 40 dia- 

 meters, carrying a reflector set at 45° in front of the 

 object-glass. The telescope is always horizontal, and 

 can turn upon its longitudinal axis as well as upon its 

 stand in azimuth. It therefore commands all points of 

 the sky, and the observer can sit in comfort, a point 

 essential for accuracy. The view of half the object- 

 glass is intercepted by a second mirror set at 45°, which 



NO. 2016, VOL. 78] 



receives light from a third, which may be parallel with 

 it or may be inclined at an angle of 10° or so, 

 roughly indicated by a pointer. The real point is that 

 at whatever angle it is set, it may be clamped infallibly 

 until the observations are complete. The box contain- 

 ing these two last-mentioned mirrors is capable of slow 

 rotation by means of a worm from the eye end of the 

 telescope, so that if, for instance, these two mirrors 

 were set at an angle of 5° and the worm were turned, 

 the observer would see successively all points in a circle 

 of 10° radius round the point to which the uninter- 

 cepted part of the instrument was directed. Suppos- 

 ing now the moon to be observed directly, and a fol- 

 lowing star, which is just too far away to be brought 

 into contact with the limb, to be seen by double 

 reflection, then a slight sweeping motion may be 

 given so as to ascertain the exact time of tangential 

 contact. Similarly a preceding star which is just too 

 near for a tangential contact may be observed until 

 the moment at which this is seen also. Actually it is 

 the star, not the moon, which is looked at directly, as 

 by that means the moon's light after two reflections 

 from unsilvered glass is sufficiently subdued to allow 

 sixth-magnitude stars to be observed in contact. The 

 designer has ingeniously introduced the equivalent of 



the spider lines of a 

 transit instrument 

 by providing a 

 slightly prismatic 

 plate which may be 

 set in any one of 

 three positions in. 

 one of the optical 

 paths, thus making 

 small but invariable 

 differences in the 

 apparent angle, so 

 that three observ- 

 ations are possible 

 for each star. 



It will be seen, 

 then, that the accu- 

 racy obtainable is 

 only limited by the 

 optical definition of 

 the telescope. 



Space is not 

 . - available for follow- 



ing the author of the paper through his discussion of 

 the observations or for referring to his excellent 

 graphic methods for shortening the calculations where 

 thev are available. It is sufficient to say that with 

 his apparatus he has found that the probable error of 

 a single observation is of the order of i'5 seconds of 

 longitude. C. V. Bovs. 



THE CAVENDISH LABOR.iTORY. 



ON October 10, 1870, the seventh Duke of Devon- 

 shire wrote to the then Vice-Chancellor of the 

 University of Cambridge, offering to provide funds 

 for a building and for apparatus for the teaching 

 of experimental physics. The building was opened 

 on June 16, 1874, when the Chancellor received the 

 thanks of the L'niversity, and at the same time ex- 

 pressed his " wish to provide all instruments for the 

 Cavendish Laboratory which Prof. Maxwell may con- 

 sider to be immediately required either in his lectures 

 or otherwise." 



Twenty years later the laboratory was increased 

 towards the south by a new building, running along 

 Free School Lane, which contained, amongst other 

 rooms, a spacious elementary laboratory. For this. 



