368 BF.XJ. PIKE'S, JR., DESCRIPTIVE CATALOGUE. 



room or place, and are well adapted for general observa- 

 tions, and performing most of the useful problems in as- 

 tronomical science. 



The basis of all equatorial instruments is a revolving 

 axis placed parallel to the axis of the earth, by which an 

 attached telescope is made to follow a star, or other celes- 

 tial bodies, in the arc of its diurnal revolution, without 

 the trouble of repeated adjustments for changes of eleva- 

 tion, which quadrants and circles with vertical and hor- 

 izontal axes require. Such an instrument is not only 

 convenient for many useful and interesting purposes in 

 celestial observations, but is essentially requisite in cer- 

 tain cases, particularly in examining and measuring the 

 relative positions of two contiguous bodies, or in deter- 

 mining the diameters of the planets when the spider's 

 line micrometer is used. The great cost of these instru- 

 ments has hitherto prevented their coming into very gen- 

 eral use, though it is one of the most pleasing and useful 

 instruments connected with astronomical science. 



The principal parts of the equatorial instrument (fig. 

 791) are : The lower or horizontal plates, the semicircle 

 of altitude, the equatorial plates, and the semicircle of 

 declination on which the telescope is mounted. The 

 lower horizontal plate is nine inches in diameter, and 

 fastened to the round head of the mahogany stand ; it is 

 divided into 360 degrees, which are subdivided into half 

 degrees or 30 minutes, and numbered from every 10 

 degrees, the upper horizontal plate is fitted to the lower 

 by a conical axis, passing through the lower plate and 

 having a large winged-screw nut on the inner side of the 

 stand, by which it may be firmly held in any position it 

 may be set. On the top of the upper horizontal plate 

 are two levels at right angles to each other, having ad- 

 justments at their ends, by which they may be accurate- 

 ly set, and by means of the three screws on which the 

 instrument is supported, the horizontal plates may be 

 accurately leveled. The semicircle of altitude is seven 

 and a half inches in diameter, is attached to a horizontal 

 axis, and supported by two standards fastened to the up- 

 per horizontal plate ; it is divided into two quadrants of 

 90 degrees each, subdivided into half degrees, and num- 

 bered from to 90 each way, a vernier reading to single 

 minutes, and a tangent-screw, by which the vertical arc 



