PROGRESS IN ASTRONOMY. 135 



(1866). While the century is clo.sing another catalogue far more stu- 

 pendous than an^'thing which could be conceived possible a few j^ears 

 ago is steadily being compiled. This we owe to the farsightedness 

 and energy of Admiral Mouchez, a late director of the Paris Obsery- 

 atorv. The work was commenced in 1892. 



The whole heayens, north and south alike, haye been divided into 

 zones, and the chief observatories on the earth's surface are T)usy 

 night after night in taking photographs of that part intrusted to 

 them. The whole heavens are thus l)eing made to write their autobi- 

 ography, and the total gain to the astronomy of the future of this 

 most priceless record can perhaps be scarcely grasped as j^et, although 

 the advantage of being al^le at any point of future time to see on a 

 photographic plate what the heavens are telling now is sufficiently 

 obvious. 



Catalogues of the stars have, of course, led to other minor cata- 

 logues of various classes of stars, ])inary, variable, and the like. In 

 the later years catalogues of stars, according to their spectra, have 

 enriched science. 



The tirst extensive catalogue of stella spectra was published by 

 Vogel. It dealt with -1,051 stars, and appeared in 1883. It has since 

 been followed b}^ the Draper catalogue, leased upon photographs of 

 the spectra, which contains a much larger number. With regard to 

 nebuh\? Herschel pul^lished his third catalogue in 1802. The last cata- 

 logue of this nature is by Dreyer (1S88). and contains 7,810 of these 

 objects. In the time of Tycho they could be eounted on the lingers of 

 one hand. 



INVESTIGATIONS OF SOMK I^MPORTANT ASTRONOMICAL CONSTANTS. 



The century has been fruitful in the determination of many numer- 

 ical values which are all important in ena])ling us to determine the dis- 

 tance and masses of the heavenly l)odies, thereb}' giving us a firm 

 grasp not only of the dimensions of our own system, but of those 

 scattered in the celestial spaces. 



To take the distances first. We nuist begin with the exact measure 

 of the earth; for this we must measure the exact length of an arc of 

 meridian or of parallel; that is, a stretch of the earth's surface lying 

 north or south or east and west, between places of which the latitudes 

 arc accurateh' known in the former case and the longitude in the latter. 

 In either case we can determine the number of miles which go to a 

 degree. Beginning at the opening of the century with an arc of 

 meridian of 2 degrees measured ])y Gauss from Gottingen to Altona, 

 the arcs of meridian have grown longer as the centurj" has grown older, 

 till, at the close, the measurement of an arc of meridian from the Cape 

 to Cairo, embracing something like 68 degrees of latitude, is being 

 mooted. 



