Penetration into Space ky Telefcopes. I5 



for the number of ftars in the clufter, and D for Its diftance, we have — Z)*; 



which, on computation, comeS out to be above iij millions of millions of millions of 

 miles ! a number which exceeds the diftance of the neareft fixed ftar, at leaft three hun- 

 dred thoufand times. 



From the above confiderations it follows, that the range for obferving, with a telefcope 

 fuch as my 40-feet refle£lor, is indeed very extenfive. We have the infide of a fphere to 

 examine, the radius of which is the immenfe diftance juft now afligned to be within the 

 reach of the penetration of our inftruments, and of which all the celeftial obje£ts vifible to 

 the eye, put together, form as it were but the kernel, while all the immenfity of its thick 

 ftell is referved for the telefcope. 



It follows, in the next place, that much time muft be required for going through lb ex- 

 tenfive a range. The method of examining the heavens, by fweeping over fpace, inftead 

 of looking merely at places that are known to contain objedts, is the only one that can be 

 jufeful for difcoveries. 



In order therefore to calculate how long a time it muft take to fweep the heavens, as far 

 as they are within the reach of my 40-feet telefcope, charged with a magnifying power of 

 1000, I have had recourfe to my journals, to find how many favourable hours we may art- 

 nually hope for in this climate. It is to be noticed, that the nights muft be very clear j 

 the moon abfent ; no twilight ; no hazinefs ; no violent wind ; and no fudden change of 

 temperature ; then alfo, (hort intervals for filling up broken fweeps will occafion delays ; 

 and, under all thefe circumftances, it appears that a year which will afford 90, or at nioft 

 100 hours, is to be called very produftive. 



In the equator, with my 20-feet telefcope, I have fwept over zones of two degrees, 

 with a power of 157 : but, an allowance of 10 minutes in polar diftance muft be made, 

 for lapping the {weeps over one another where they join. 



As the breadth of the zone* may be increafed towards the poles, the northern hemi- 

 fphere may be fwept in about 40 zones; to thefe we muft add 19 fouthern zones 5 then, 

 59 zones, which, on account of the fweeps lapping over one another about 5' of time in 

 right afccnfion, we muft reckon of 25 hours .each, will give 1475 hours. And, allowing 

 100 hours per year, we find that, with the 20-feet telefcope, the heavens may be fwept ia 

 about 14 years and ^. 



Now, the time of fweeping with different magnifying powers will be as the fquares of 

 the powers ; and, putting p and t for the power and time in the 20-feet telefcope, and P 



= J 000 for the power in thc40, we fhall have />»:/:: P': = 59840. Then, ma- 



- . • ' t 



* D — 11765475948678678679 miles, 



king 



