penetrating into Space by Telescopes, 85 



In the equator, with my 20-feet telescope, I have swept over 

 zones of two degrees, with a power of 157 ; but, an allowance 

 of 10 minutes in polar distance must be made, for lapping the 

 sweeps over one another where they join. 



As the breadth of the zones may be increased towards the 

 poles, the northern hemisphere may be swept in about 40 zones : 

 to these we must add 19 southern zones ; then, 59 zones, 

 which, on account of the sweeps lapping over one another about 

 5' of time in right ascension, we must reckon of 25 hours each, 

 will give 1475 hours. And, allowing 100 hours per year, we 

 find that, with the 20-feet telescope, the heavens may be swept 

 in about 14 years and J. 



Now, the time of sweeping with different magnifying powers 

 will be as the squares of the powers ; and, putting p and t for 

 the power and time in the 20-feet telescope, and P == 1000 for 



the power in the 40, we shall have p* : t : : P z : —?- = 59840. 



Then,, making the same allowance of 100 hours per year, it 

 appears that it will require not less than 598 years, to look with 

 the 40-feet reflector, charged with the abovementioned power, 

 only one single moment into each part of space; and, even 

 then, so much of the southern hemisphere will remain unex- 

 plored, as will take up 213 years more to examine. 



Slough, near Windsor, 

 June 20, 1799. 





