of ihe late Mr. Ra}n<i(len. 259 



much further improved bv Ramsden. lie first suppressed 

 th^ endless screw, which by pressing on the centre destroyed 

 its precision. He placed the centre of gravity on the centre 

 of the base, and caused all the movements to take place in 

 every direction. He pointed put the means of rectifying the 

 instrument in ail its parts; and he applied to it aveiy inge- 

 nious small machine for measuring or correcting the cfl'ect 

 of refraction. This invention is much anterior to that given 

 bv Mr. DoUond in the Philosophical Transactions. Mr. 

 Ramsden had a patent for this kind of equatorial. Theho- 

 noural)le Stewart Mackenzie, brother of the earl of Bute, 

 the friend and patron of Mr. Ramsden, wrote a description 

 of this machine, which has been printed. But Mr. Ramsden 

 does riot always strictly adhere to the description : his in- 

 ventive genius rarelv allows him to construct the same in- 

 strument many times in the same manner ; and it often 

 happens that he breaks to pieces instruments which have 

 cout a great deal of labour, if they are not as correct as he 

 wishes. 



The largest equatorial instiiniient ever constructed is that 

 destined for Sir George Shuckbnrgh, on which Mr. Rams- 

 den has been employed for nine years. The circle of inclina- 

 tion is four feet in diameter, so that observations can be made 

 nearly within a second. The telescope is placed between six 

 pillars, which form the axis of the machine ; and the whole 

 turns around two pivots resting on supporters of masoi>- 

 work. 



The transit instrument is employed in all the large ob- 

 servatories of Europe; but Mr. Ramsden has added to it 

 several improvements. He invented a method of illumina- 

 ting the wires, by making the light pass along the axis of 

 the machine. The reflector is placed in the inside, and ob- 

 liquely in the middle. He did not lessen the aperture of the 

 object-glass : and as the light passes through a coloured 

 prism, which may be moved at pleasure, the light may be 

 increased or diminished. For adjusting this essential instru- 

 ment, Mr. Ramsden invented a method which supersedes 

 the use of a spirit-of-wine level, on which he sets no 

 value, because it does not give that exactness which it is 

 always his aim to obtain. He suspends a thread and 

 plunnnct before the tclesco])e placed vertically : this thread 

 passes over two points, which are marked on two pieces fixed 

 (.ne above and the, other below the telescope, and one of 

 which has a small motion. The thread is absolutely de- 

 tached from the telcscfipe ; and when it corresponds on the 

 same points in the two different situations of the telescope, 

 K 2 the 



