THE GREAT STAR MAP 433 



The scale of a star map depends essentially on the length 

 of the telescope used in making it. The little instrument 

 with which the Harvard Sky was made is only about a foot 

 long, whilst those used for the Great Star Map are about 1 1 

 feet. The surface of the representation increases, of course, as 

 the square of the linear dimensions, so that we might fairly 

 expect the weight of the plates to be increased in the ratio 

 121 to I ; and since the plans for the Map involve covering 

 the whole sky twice over, we must double this, getting 242. 

 Now the ratio of 3 tons to 30 lb. is 224 to i, so that the 

 weight is adequately explained by the increase in scale. If we 

 use the same factor (242 or thereabouts) to get an idea of the 

 cost of the plates forming the large map, from the Harvard 

 $15 (which however is less than the actual cost of the Harvard 

 Sky, though it is generously offered for sale at this figure), 

 multiplying 15 by 242 we get 3,600 dollars or ;!f7oo. But the 

 actual cost is greater than this because plate glass has been 

 used and two edges of each plate have been specially ground ; 

 probably ;^ 1, 500 or ;^2,ooo is not too much to put down as the 

 cost of the plates. But this after all is only a very small portion 

 of the real cost of the Map, which arises chiefly from the work 

 done on the plates after they have been taken. In order to 

 expedite comparison with other plates to be taken in the future, 

 the present positions of the star images on the plates are being 

 carefully measured and the measures printed. How much 

 shall we set down for this ? It is impossible to make an 

 accurate estimate since circumstances vary so much with 

 nationality, but some rough idea may be gathered from the 

 experience of Oxford, where the measures have been made and 

 printed. Even here there are difficulties, as will appear from 

 the following statement of a generously superior limit to the 

 total cost : 



Total Possible Cost of the Oxford Astrographic Catalogue, 



£ 



Telescope given by Dr. Warren De la Rue .... 600 

 Maintenance of University Observatory for twenty years, in- 

 cluding Assistance 13,000 



Salary of Professor for twenty years 18,000 



Government Grant (from the fund administered by Royal 



Society) in the years 1896-1910 1,200 



Printing (shared between the Government and the University) 1,200 



Total .... 34,000 



