threads exactly equal to an inch. In short, he can ensure equality and fine- 

 ness, but can not confer upon the threads particular, definite, and exact di- 

 mensions. Nor is it necessary that this object should be attained, even were 

 it practicable. The observer being furnished with the instrument, each divis- 

 ion of which means something, can find out from the heavens what that some- 

 thing is. 



This is very easily accomplished. Supposing the observer to be provided 

 with a clock or watch which beats seconds (the extreme accuracy of a chro- 

 nometer is not here required), let him direct the telescope as nearly as 

 he can to that point of the southern meridian where the equator intersects 

 it. Very extreme accuracy is not required in this adjustment. Let him 

 then place the fixed and moveable wire in a vertical position, and bring- 

 ing them to coincidence, let him separate them by giving ten complete 

 revolutions to the screw. Let him then watch the moment when any par- 

 ticular star is seen on the first wire. This he can determine (by listening 

 to the ticking of the clock), to nearly half a second. Let him then wait 

 until, by the diurnal motion of the heavens, the star comes upon the second 

 wire, and observe the time it arrives there. He will then know the time the 

 star took to pass from one wire to the other. But since the firmament makes 

 a complete revolution (360) in twenty-four hours, it moves at the rate of 

 15 per hour, or 15" per second of time. Let us then suppose that the 

 time which the star took to pass from the one wire to the other was 22^ 

 seconds. The space corresponding to this would be 22.5 x 15=337". 5 ; 

 which would therefore be the space between the wires corresponding to the 

 revolutions of the screw. The space corresponding to one revolution would 

 then be 33". 75, and the space corresponding to one division of the head of 

 the screw would be 0.33", or one third part of the second of a degree. 



If the observer be not provided with a clock, or can not conveniently use 

 one, if he has it, he may still accomplish the object. Let him in that case 

 direct the instrument to the sun at or near noon, and let him adjust the moveable 

 and fixed wires so that they shall just touch the upper and lower limb of the 

 sun, the position of the wires being horizontal. The space between the wires 

 will then correspond to the apparent diameter of the sun. By reference to 

 the nautical almanac (with which he ought always to be provided), he can 

 ascertain the apparent diameter of the sun at the time of his observation. 

 Suppose that this is found to be 31', 56", or 1916". Then the interval be- 

 tween the wires will be 1916". Let the screw be turned until the moveable 

 wire coincides with the fixed wire, and suppose the number of turns and parts 

 of a turn necessary to effect this is found to be GO complete revolutions and 

 12 divisions, or 6,012 divisions; then 1,916 divided by 6,012 gives 0".3186 

 as the value of each division, or 31. "86 of each complete revolution. 



We shall now conclude. Enough has probably been said to encourage the 

 amateur observer, and to set him on the track, by the pursuit of which he may- 

 obtain much personal gratification, some reputation in the community of sci- 

 ence and render himself useful in the promotion of knowledge. If he begin 

 he will not rest content with these hints, but will call to his aid other more 

 ample and detailed instructions, to be found in the works already referred to, 

 and in the memoirs published by the different scientific bodies of Europe. 



VOI..II.-23 



