1863.1 155 [Coppee. 



hend them. To give himself facility in observing, he had studied 

 under Professor Airey, the Asti'onomer Royal of England at Green- 

 wich ; and to understand the scientific relations of astronomy as they 

 appear in the cosmogony of the universe, he had investigated those 

 sister sciences which, while they are elements of the great subject, 

 come forward in their progTess of development to cast their tribute 

 at the feet of Him who dictated the record of Moses. 



As a mechanical inventor, he may be best presented by placing in 

 this connection, some account of the principal instruments which he 

 created for facilitating observations. 



The following description of the Declinometer is furnished through 

 the kindness of Mr. Gr. W. Hough, the astronomer in charge of the 

 Dudley Observatory : 



" Method invented by Professor Mitchel for determining diflfer- 

 ence of declination. 



" The apparatus for observing difference of declination consists of 

 the following : 



" To the axis of the transit telescope is attached a metallic arm of 

 sixty inches in length ; in the lower end of this arm is screwed a 

 cylindrical pin, one-eighth of an inch in diameter, at right angles to 

 the arm and parallel to the supporting axis of the telescope. This 

 pin has a notch or groove (of the form which would be generated by 

 placing the vertices of two isosceles triangles together, and revolving 

 about the perpendicular), cut in the middle. 



" At a distance of twenty-three inches from the pin, and in the 

 same horizontal plane, is mounted in Y's a small telescope of six in- 

 ches focal length. The supporting axis of this telescope is parallel 

 to that of the transit. Underneath the centre of the small telescope, 

 and connected with it, is a short arm two inches in length, and by 

 means of a joint, a rod is connected with the pin before mentioned. 



" Now when the transit telescope is moved in zenith-distance, an- 

 gular motion is given to the small telescope, by means of the long- 

 arm and connecting-rod. 



" The amount of this motion is read from a scale, placed at a dis- 

 tance of fifteen feet, and divided to single seconds of arc. It will, of 

 course, be understood that we must have some object in the focus of 

 the small telescope with which to compare the divisions of the scale. 

 We use either a cross formed by the intersection of two spider's 

 webs, or a single horizontal wire. 



" In case we wish to observe a zone of greater width than the ex- 

 tent of the scale (30'), we have a number of pins, at distances of 



