174 ANNUAL OF SCIENTIFIC DISCOVERY. 



From this principle the revolving disk resists all angular change of position 

 in directions perpendicular to its plane ; but it offers no resistance to any mo- 

 tion in that plane. Thus a free revolving disk in gymbals externally turn- 

 ing on pivots horizontally resting on supports fixed to the deck, will suffice 

 to preserve the telescope from all deviation due to pitching and rolling. The 

 addition of another disk, freely revolving in a vertical plane, whose external 

 pivots turn vertically in a frame attached to the top of the former internal 

 frame, the upper pivot projecting through it, and carrying a small platform 

 for the telescope, and the whole, of course, balanced below, will preserve 

 the telescope from any lateral deviations of the ship. And the combination 

 of the two will give a plane retaining its parallelism against all three causes 

 of disturbances. But under favorable circumstances this last cause of dis- 

 turbance is but small; so that this addition may be often of little impor- 

 tance. The invariable platform of this revolving apparatus may then 

 equally be applied to support either a telescope, or the artijicial horizon 

 before mentioned, whether simply, or in conjunction with the sextant or 

 reflecting circle. By a mere enlargement of the scale of the machine, the 

 same stand which curries the telescope might be made to carry the observer 

 also, which would be a material convenience for any nice observations. But 

 the essential condition is the preservation of perfect equilibrium about the 

 centre of motion. Now, if this were secured by proper compensation for 

 the observer in one position, the slightest change of position on his part 

 would vitiate that arrangement. The observer, instead of being an extrane- 

 <"'s source of disturbance incapable of producing any impression on the bal- 

 anced and revolving system, now becomes a part of it, and thus impresses 

 upon it a fresh motion arising from every slight change of posture, which 

 alters the exact position of the centre of gravity of the whole. This effect, 

 however, would not manifest itself directly; but being compounded with the 

 rotation, would show itself in a precessional motion, fatal to the stability of 

 the direction of the telescope. A modified arrangement to meet this contin- 

 gency has (which after all is not one of great importance practically, though 

 interesting theoretically) been devised by Prof. Smyth. To complete the 

 whole, Prof. Smyth has carefully investigated the best form of a train of 

 wheels for communicating the rotatory motion, and has also considered the 

 question of the best moving power to be used; which he finds, after many 

 trials, to be that of water : which is but brought to bear by a peculiarly beau- 

 tiful and simple form of the turbine. Thus, taking a summary view of the 

 whole subject: by direct consequence, from the simplest acknowledged 

 mechanical principles, the gyroscope, when its equilibrium is slightly dis- 

 turbed, demonstrates the precession of the equinoxes; explains the boom- 

 erang; and sustains itself in the air against gravitation. When its equilib- 

 rium is undisturbed it exhibits to the eye the actual rotation of the earth; 

 and when restricted to one plane it acts as a magnetic needle without mag- 

 netism, or spontaneously rotates in parallelism with the earth. To these 

 remarkable, diversified, and somewhat paradoxial applications, we have 

 now to add another of far higher utility, that it gives perfect stability for 

 the nicest astronomical observations on board a ship pitching and tossing 

 with every way and gust of wind. 



ON A NEW ARTIFICIAL HORIZON. 



At a recent meeting of the Royal Astronomical Society, Professor Baden 

 Powell read a paper by Prof. C. Piazzi Smyth, on the above subject. After 



