OF ARTS AND SCIENCES. 271 



saved, and the search could be prosecuted with greater rapidity. la 

 the autumn of 1877, I devised for this purpose the piece of mechanism 

 which I am about to describe. 



In sweeping over the zones in right ascension, the clock-motion and 

 sector are, of course, detached from the polar axis. The arc of the 

 sector is to be graduated, as the right-ascension-circle is graduated ; 

 it need be only a continuous graduation of hours and parts thereof. 

 Sliding upon this graduation, or adjacent to it, are two metallic 

 vernier-like pieces, both of which are furnished with screws for clamp- 

 ing to any part of the graduated sector-arc. Each of these verniers 

 carries a projecting metallic point, attached to it on a line joining the 

 centre of the polar axis and the zero point of the vernier. Revolving 

 freely about the polar axis, and adjacent to the sector, is a collar, 

 carrying a projecting arm the end of which will just touch the metallic 

 points attached to the verniers. This collar has a screw for clamping 

 it to the polar axis, just as the sector has. And, moreover, electric 

 apparatus is so disposed that, whenever the end of the projecting arm 

 comes in contact with either of the metallic points attached to the 

 verniers, a telegraphic sounder shall beat, or an electric bell shall 

 ring. 



The apparatus is now complete. Its use is as follows : — 



By means of the graduation on the sector, the two verniers con- 

 tiguous to it are set at a distance apart equal to the length of the zones 

 to be searched. The sector is then undamped from the polar axis 

 and connected with the clock, and the clock set a-goiug. The tele- 

 scope is then set to the right ascension of one end of the zones to be 

 searched, and the projecting arm is at the same time brought into 

 contact with the metallic point of the corresponding vernier, and 

 clamped to the polar axis in that position. After the declination- 

 circle is set, the whole instrument is ready for the search, and it is 

 not necessary to remove the eye from the telescope, as the click of 

 the sounder or the ring of the bell apprises the ear of the observer 

 whenever the telescope reaches either limit of the zone in right 

 ascension. 



I venture to predict the usefulness of this piece of apparatus in the 

 search for intramercurial planets during total solar eclipses, when the 

 available time is a minimum, and the area to be swept over is com- 

 paratively quite extended. I hope, also, for its successful application 

 in orbit-sweeping ; it was in connection with the execution of such a 

 search that the apparatus was devised. Furthermore, in the event of 

 search for an object so faint as to require a very large telescope, I 



