OBSERVATORIES AND INSTRUMENTS. 



683 



stars, for one hour of right ascension : the num- 

 ber of stars taken within such limits is often as 

 great as two hundred, and, in the region of the 

 galaxy, sometimes amounts to four hundred 

 and eighty. At the end of an hour, the pen, 

 which has swept but a portion of the entire 

 length of the cylinder, can be shifted over a 

 new belt of the record sheet, and another 

 hour's observations taken. Further, however, 

 as each dot is made on the record sheet, a si- 

 multaneous record is also made on the work- 

 ing chronograph, and which gives the time to 

 the one-hundredth part of a second ; and for 

 the exact declination, an assistant reads the 

 declinometer scale to the five-tenths of a 

 second. Thus are obtained at once a complete 

 catalogue of the exact positions of the stars, 

 and also a perfect map of the given part of the 

 heavens. The apparatus can be adapted to 

 any telescope. Of course, by showing the fact 

 of change of position in any star, or the en- 

 trance of a new one within a given zone, upon 

 taking sooner or later a second chart of it, this 

 apparatus affords great facility both for the 

 finding of known asteroids and the detecting 

 of previously undiscovered ones. Again, if 

 records have been obtained of several contigu- 

 ous zones, these may be subsequently pricked 

 through upon a single sheet, or otherwise 

 transferred, thus producing star-maps of larger 

 size. 



An Aluminium Bronze Transit Axis. In 

 this CYCLOPAEDIA, 1862, will be found, under 

 the head of CIIEMISTIJY, an abstract of a paper 

 by Lieutenant-Colonel A. Strange, in relation 

 to the qualities of various bronzes of alumin- 

 ium and copper, and the probable value of some 

 of these for the construction of instruments for 

 scientific purposes. 



April 12, 1865, Col. Strange presented be- 

 fore the Royal Astronomical Society a paper in 

 the first part of which he detailed the results 

 obtained in the way of castings of the bronze, 

 and in particular its successful application to 

 the construction of a transit axis. He now 

 states that the alloy has been extensively em- 

 ployed, and that it has answered all his expec- 

 tations, with a single exception ; namely, that 

 in consequence of its tarnishing more than was 

 anticipated, and of the difficulty of producing 

 a surface absolutely free from defects, the metal 

 is not found so suitable as was hoped for receiv- 

 ing graduation. In the early experiments, also, 

 there was a difficulty in securing large and 

 complex castings, owing to the metal's running 

 sluggishly into the mould and solidifying very 

 quickly ; but this had been overcome. In con- 

 firmation, the author exhibited an axis formed 

 of the bronze, and made for a transit-instru- 

 ment, of which the focal length was to be five 

 feet. The length of the axis between bearings 

 was 33$ inches ; the central cube nine inches 

 by the side ; the average thickness of the met- 

 al but 0.15 of an inch; and the total weight 

 only 56 pounds. It was cast hollow, and in 

 Dne piece, being made to include also an inge- 



nious system of internal strengthening webs: 

 the work was executed by the Messrs. Cooke, 

 of York. From the effect of these webs, in con- 

 junction with the rigidity of the alloy itself 

 the latter being three times as rigid as gun- 

 metal this was believed to be the stiffest tran- 

 sit axis hitherto made. In the second part of 

 his paper, Col. Strange considered the adjust- 

 ments of the instrument. He endorsed Mr. 

 Cooke's suggestions, in respect to suspending 

 one or more levels to the tube of the telescope 

 itself, these watching the telescope instead of 

 the pivots ; and in respect also to the covering 

 of the mercury surface, ordinarily relied on in 

 collimating, with a disc of glass having accu- 

 rately plane and parallel surfaces. 



The ChronograpMc or Automatic Method, in 

 Transit Observations. The adoption of the 

 automatic method for the recording of transit 

 observations is steadily extending. It has been 

 well remarked, that there is a degree of profi- 

 ciency beyond which no amount of training 

 can carry human skill; and that, when this 

 has been reached, it is proper to give the re- 

 sources of the intellect a new direction, in the 

 devising of mechanical contrivances which 

 shall accomplish the desired end with a precis- 

 ion not directly attainable by the human or- 

 gans. M. Faye insists on the advantage of sub- 

 stituting largely the work of automatic mechan- 

 isms in lieu of that of the observer's hand, as a 

 means of avoiding the errors of the senses, and 

 also [of diminishing, at least] those " personal 

 errors " which have no fixed value and which 

 can scarcely be corrected. 



In the Monthly Notices of the Royal Astro- 

 nomical Society, for May, 1864, appears a paper 

 " On the Probable Error of a Meridional Tran- 

 sit-Observation, by the 'Eye and Ear' and 

 Chronographic Methods" communicated by Mr. 

 E. Dunkin. The author treats at length of the 

 sets of observations chosen by him for exami- 

 nation, and of his modes of discussing them 

 particulars for which space cannot here be al- 

 lowed. His results are, however, evidently 

 drawn, as he states, from "a great mass of 

 computation ;" and the principal of them are 

 the following : 



1. In " eye and ear " observations, the proba- 

 ble error of a Greenwich transit observed in 

 1853 over one wire is 0.'078, while that of a 

 complete transit over the seven wires is O.*029. 

 In chronographie observations, the probable 

 error of a Greenwich transit observed in 1857 

 over one wire is 0.*051, and that of a complete 

 transit over the nine wires is 0. S 017. 



3. In the eye-and-ear transits, only, for stars 

 whose north polar distance is greater than 60', 

 it would seem that the probable error of a tran- 

 sit increases slightly as the north polar distance 

 increases. 



4. In eye-and-ear transits, the personal dis- 

 cordances are liable to a considerable variation 

 between the different observers; in chrono- 

 graphie transits, the differences between the 

 observers are comparatively small. The gen 



