Portable Equatoreals. 167 



The equatoreal, then, is a telescope mounted in such a 

 manner as to enable the observer to follow the diurnal motion 

 of a celestial object by one uniform movement round an axis. 

 This axis produced would necessarily touch, so to speak, the 

 heavens at the pole, the altitude of which equals the latitude of 

 the place. Any method by which the observer can readily 

 place this axis in such a position as to point to the pole in the 

 heavens will place his instrument in adjustment and ready 

 therefore for use ; but in practice this is not so easy a matter 

 as it might at first sight appear to be. In order that the 

 telescope shall be correctly pointed to an object, the declination 

 or north polar distance of the object must be known, and the 

 index set to the proper division. The movement on the axis 

 referred to will then be sufficient to ensure the telescope's 

 following that object by the axial motion already referred to 

 without again touching the motion for declination. A little 

 consideration will show such matters very clearly. This di- 

 urnal axial motion is performed by a clock in the larger equa- 

 toreals, so that the declination circle being once set, the object 

 remains in the field of view as long as it is above the horizon, 

 without any further help from the observer. Of course this is 

 on the assumption that the declination of the object in question 

 does not itself change during the interval. The clockwork move- 

 ment is a luxury which the possessor of the portable equa- 

 toreal must not hope to realize. He must be satisfied to move 

 his instrument by hand, but when once he has carefully set the 

 declination-circle he will find the following of the object by 

 the one motion only, even by hand, a very great advantage 

 compared with the "fishing" kind of use of the telescope 

 which is necessary in other cases. 



Several years ago the writer amused himself by trying to 

 find out the simplest kind of construction capable of converting 

 an ordinary pocket telescope into a portable equatoreal. The 

 result is described in Recreative Science. The two movements 

 used in the construction of that very useful little thing, 

 a brass " clip," with an iron screw at its base to enable the 

 observer to secure his telescope to a post, tree, window-sash, etc., 

 with perfect steadiness, being the same in principle as those of the 

 large equatoreals, suggested the using of such clip equatoreally. 

 To do this it was necessary to graduate the two little circles 

 where the motion is visible. There was some difficulty in this, 

 but the labour was well bestowed, and the result was perfectly 

 satisfactory. Thirteen years' occasional use of the instrument 

 has quite proved this. It never fails in enabling the observer 

 to find an object in the field of view by day or night. But it 

 is in the daytime when the little instrument desires to display 

 its powers most, when an object is found by it which could not 



