Coppee.] Jgg [March. 



30' apart, mounted in the arc of a circle whose radius is equal to the 

 length of the long arm. We readily pass from one pin to another, 

 by lifting one end of the connecting-rod, and attaching it to a differ- 

 ent one. The divisions on the scale can easily be read by estima- 

 tion, to two-tenths of a second of arc. 



'' The time required to read the scale is much less than that em- 

 ployed in reading one microscope, since at the same transit of an 

 equatorial star we can make from ten to fifteen bisections and read- 

 ings. As I have found one reading of the scale nearly equal to four 

 microscopes, it follows that if we employ the same time in the ob- 

 servation of an object with the Declinometer that we do when we 

 use the Circle, our results in the former case will be superior to the 

 latter in a large ratio. 



" The Zone observations with the Declinometer have been made 

 mostly for the investigation of the source and amount of error due to 

 this method. From a comparison of the observations with those 

 made in the ordinary way, I find the probable error, on a single ob- 

 servation, falls within the limits of accuracy usually assigned to ob- 

 servations made with the Meridian Circle. One great advantage 

 lies in the fact that many bisections and readings can be made at the 

 same transit, and in this way eliminating the ordinary errors of 

 observation. You will understand the rapidity with which work can 

 be done by this method, when I state that more than two hundred 

 stars have been accurately observed in one hour; and were they 

 equally distributed, twice that number could easily have been taken. 



" This instrument is one of the great inventions of our late and 

 lamented directoi". Professor Mitcbel, and is the only one in the 

 world. 



" From observations made during the last two years, and a careful 

 discussion of the results, I have arrived at the conviction that there 

 is no other hiown method equal to it, for rapidity and accuracy, in 

 the cataloguing of stars." 



Professor Mitchel's remarkable mechanical skill, bis quickness to 

 perceive diflSculties, and the readiness with which he devised and 

 applied the remedies, are further admirably illustrated in his appa- 

 ratus for recording time by means of the electro-magnetic telegraphs. 

 These are now in use in the Cincinnati and Dudley Observatories. 

 His was the first thorough solution of this important problem in 

 instrumental astronomy. The following account of this apparatus is 

 in Professor Mitchel's own words : 



