482 Rev. B.Powell, [Feb. 26, 



The principle of fixity of the plane of rotation had been 

 universally recognised in theory ; and it could not have been 

 doubted that in proportion to the momentum acquired by giving 

 immense velocity to the rotating mass, this constancy would be 

 more vigorously displayed ; yet perhaps few were prepared for the 

 actual result as exhibited by M. Foucault : even when the princi- 

 ple was acknowledged, nothing could seem more astonishing than 

 the obstinate resistance of the disk to any inclination from its 

 onguial plane of rotation ; which no ordinary degree of force would 

 overcome. This principle is that chiefly referred to in the inven- 

 tions about to be described, where the effect depends essentially on 

 the great amount of resistance thus offered to any angular motion 

 impressed by an extraneous cause on a perfectly balanced revolv- 

 ing heavy disk. 



When we consider the vast amount of precautions taken by 

 astronomers for securing the stability of their instruments, and the 

 careful plans adopted for guarding against every imaginable cause 

 of disturbance on land, it may seem surprising that even any 

 attempt should be 'made to carry on such operations at sea. Yet 

 it is a matter of necessity : some observations must be made for 

 determining the place of the ship, on which its safety depends; and 

 other cases often occur when phenomena of great value to astro- 

 nomy, — the science without which the ship could not be navigated, — 

 are required to be observed at sea ; or may perhaps only be visible 

 at positions out on the ocean. The most important of these obser- 

 vations are those of the altitudes of *the heavenly bodies, on which 

 depends both the determination of the latitude, and the correction 

 of time essential to finding the longitude; and for this purpose 

 there is a necessity for a well defined horizon, which it is often 

 impossible to obtain from the state of the atmosphere in its lower 

 parts, though the sun or star can be distinctly seen above, and this 

 more especially at night ; yet the safety of the ship may essentially 

 depend on such an observation. 



Hence various plans have been resorted to for obtaining an 

 artificial horizon. Simple reflection from the surface of a liquid 

 can hardly ever be practicable, on account of the motion of the 

 ship, though it is the usual substitute on land ; by the reflected 

 image, seen as much below the true horizon as the object is above 

 it. 



The most celebrated attempt to substitute some other principle, 

 was an application of rotatory motion, devised by the late Mr. 

 Troughton, in 1820. It consists in causing a disk, truly balanced 

 on a fixed pivot, to spin round with great velocity, so as to keep up 

 its motion during the time required for an observation, known by 

 the name of " Troughton's top." The disk carries a plane reflector 

 on its upper surface ; and being a cylinder hollowed out at its 

 lower end, and the point of support within, the centre of gravity is 

 thrown below, so that it is in stable equilibrium when at rest. The 



