682 



OBSERVATORIES AND INSTRUMENTS. 



vals, a thin blade of mica is so fixed to a pro- 

 longation of the ordinary crutch that at each 

 oscillation of the pendulum it cuts the mercury 

 wire, thus interrupting the electric current and 

 transmitting the required signal. By varying 

 the width of the plate of mica, the duration of 

 the break can be regulated. A time-gun has 

 also been set up recently on the Admiralty 

 Quay at St. Petersburg, to be fired at midday 

 by an electric current sent from the Pulkowa 

 Observatory. 



Machine for Cataloguing and Charting Stars. 

 Prof. G. W. Hough, Director of the Dudley 

 Observatory, Albany, describes in the American 

 Journal of Science, vol. xxxviii. (date of Sep- 

 tember, 1864), a machine invented by himself, 

 for the purposes indicated in the above title. 

 He alludes in the outset to the first suggestion, 

 in 1848, of the application of electricity to the 

 recording of astronomical observations, the re- 

 sult being the early construction of chrono- 

 graphs by various persons. All these had for 

 their object the recording of one ordinate only 

 of a star's position at a given moment that, 

 namely, in right ascension (celestial longitude) ; 

 except that the late Prof. O. M. Mitchell had 

 also made some experiments in the way of 

 electric recording of declinations (celestial lat- 

 itude). Again, in determining difference of 

 declination as well as time of transit, astron- 

 omers have almost invariably used the telescope 

 in a fixed position, and depended upon a dia- 

 phragm or scale placed in the focus. Prof. 

 Mitchell had, in 1849, devised a plan by which 

 the angular motion of the telescope when 

 moved in zenith distance, magnified by me- 

 chanical means, was made to show the differ- 

 ence of declination ; and this principle also 

 enters as one of the elements in Prof. Hough's 

 apparatus. 



The work heretofore of constructing star- 

 charts, by laying down by hand the positions 

 as given in a catalogue, has been extremely 

 difficult and tedious. Mr. Hough became im- 

 pressed with the desirableness of making the 

 star-map in the very process of observing for 

 correct positions. The accomplishment of this 

 important purpose, at once securing great accu- 

 racy and saving labor and time, has been at- 

 tained in. the apparatus now to be described. 

 In this, the record of the places of stars ob- 

 served is made by the point of a hollow cylin- 

 drical steel pen : the place, at any moment of 

 the pen over the sheet of paper receiving the 

 record the latter having been preparatorily 

 coiled about a horizontal cylinder directly over 

 which the pen rests is determined contin- 

 ually by two movements ; namely, first, by the 

 steady revolution, once every hour, of the cyl- 

 inder itself; and secondly, by a lateral move- 

 ment given to the pen through mechanism 

 connecting it with the clamp arm of the tele- 

 scope, viz., a horizontal arm which carries 

 the pen, a compound lever giving motion to 

 the former, and a horizontal rod and upright 

 bar by which the connection with the clamp 



arm already mentioned is completed, while at 

 the same time these parts serve further to 

 magnify in the record the angular motion given 

 to the telescope in zenith distance. 



The cylinder carrying the record paper is 

 ten inches in diameter and six inches long, and 

 is made to revolve (as placed) from west to 

 east, by means of clock mechanism and a half- 

 seconds pendulum. Thus, the apparent regular 

 movement of the pen along the paper, pro- 

 duced by the turning of the cylinder, corre- 

 sponds to differences of right ascension, and of 

 time ; while the lateral movement that in the 

 direction of the axis of the cylinder imparted 

 to the pen, corresponds to differences of decli- 

 nation. The electrical recording mechanism is 

 simple : it consists of an electro-magnet, its 

 helix of course being part of the circuit of a 

 galvanic battery, while the magnetic condition, 

 when induced in the soft-iron magnet, is made by 

 means of the armature to operate a horizontally 

 placed arm, upon the end of which is a horizontal 

 cross-piece standing parallel to the axis of the 

 cylinder and running its whole length : the de- 

 pression of this cross-piece by action of the 

 magnet forces down the arm which carries the 

 pen, over whatever part of the length of the 

 cylinder the latter may stand. To make a 

 record, it is only necessary to press a key by 

 which the circuit through the coil of the elec- 

 tro-magnet is closed : the cross-piece is thus 

 caused to strike a blow on the arm carrying 

 the steel pen, so that at the instant a small dot 

 is made on the record sheet covering the 

 cylinder. For some further details in reference 

 to the mechanism and means of modifying its 

 action, the reader must be referred to the orig- 

 inal account. 



For stars of magnitude greater than the 

 ninth, of which generally not more than three 

 or four will have to be recorded in a night, the 

 magnitudes are specially recorded by an as- 

 sistant. The magnitudes 9, 10, 11, 12, and 

 13, are distinguished in the record by introduc- 

 ing for each beneath the point of the pen just 

 before recording, and as may be required, the 

 proper one of five strips of differently colored 

 paper : each of these magnitudes is thus known 

 by a particular color imparted to the dot 

 which records the star's place. For the 14th 

 magnitude the smallest observed the dots 

 are left colorless, and are not specially marked. 

 The strips of colored paper are so placed on 

 an arm moving about a vertical axis, that the 

 observer can, without removing his eye from 

 the telescope, bring each as desired under the 

 pen. 



As fast, then, as the stars enter the field of 

 the telescope, they are brought to the intersec- 

 tion of a horizontal and a vertical wire, when 

 the electric circuit is closed and the record 

 made; and thus constantly sweeping with the 

 telescope a zone of 10' to 12' width (the great- 

 est usually practicable), the result obtained 

 during one revolution of the cylinder is a " fac- 

 simile " copy a chart of the given zone of 

 i 



