560 yfccoiait of the Life and Laloi/rs 



the observer is certain that the axis is horizontal, as you 

 have remarked in your Astronomy. What is newer and 

 more ingenious in Mr, Ramsden's method is, tliat the thread 

 and plummet sometimes pa^s over the images only of the 

 points which are formed in the focus of a lens, because he 

 is sometimes obliged to remove the thread to a considerable 

 distance from the instrument and from the points ; but the 

 exactness is not lessened, and there is no parallax. 



Ramsden's meridian telescopes, such as those at Blen- 

 heim, at Manheim, at Dublin, and such as those made 

 for the observatories of Pans and C^otha, arc also reniarkable 

 for the excellence of the object-glasses. Mr. Usher,, in a 

 letter from Dublin, says that he can see in the open day- 

 stars of the fourth magnitude, and tho-e of the third very 

 near their conjimctioa with the sun. The^e telescopes arc 

 eight feet in length. I had the satisfaction to obtain one 

 of fiyc feel for my observatorv of l'alern)o ; but it is so good, 

 and the sky is so serene, that 1 hope to have the same ad- 

 vantage in my observations. 



'i"he mural quadrant is the most important of all the astro- 

 nomical instruments, and xVJr. Kanisdcn has distinguished 

 himself here also bv the exactness of his divisions : he 

 places the thread and plummet l)ehind the instrument, ia 

 order that it may not be necessary to remove it w hen obser- 

 vations are made near the ^jenith. }Tis method of illuminat- 

 iijg the objccl'glass and at the same time the divisions, and 

 of suspending the telescope, is also new, and consequently 

 more perfect. In those of ei<;ht feet which he made for 

 the observatories of Padua and Vilna, and which Dr. Mas- 

 kelyne examined, the greatest error docs not exceed tAvo 

 seconds and a half. One for Milan is in a great state of for- 

 wardness, and is of the same size. 



The mural quadrant of the duke of Marlborough atBlen- 

 hein), which is six feet, is one of the instruments which, 

 vou and I admired. It is fixed to four pillars which turn 

 on two pivots, so that the instrument may be placed nortli 

 aj-id south in a minute. This instrument is as beautiful as 

 pexfect, and no one deserves more to possess it than his 

 grace the duke of Marlborough : professed astrononiers are 

 not more zealous, assiduous, or correct. It was for this 

 noble instrument that Mr, Kamaden invented a method of 

 rectif)'ing the arc of go degrees, respecting which an abk 

 astronomer h.id started some difficulties; but with a hori- 

 zontal thread, and a thread and plummet forming a kind of 

 cio^s which does not touch the ipiadrant, he ..howed hinx 

 that thece wag not ai second of eiror in 90 degrees ; and that: 



the 



