1 24 Transactions. 



the concave one, and thus by simply turning the plane reflector on its axis we 

 are saved the cumbrous alternative of moving the whole tubular length of the 

 telescope in order that it may point to the object to be observed. In large 

 instruments this must be a very important desideratum. Let us suppose a 

 telescope twenty feet in diameter : ordinarily this would require tubing at 

 least 120 feet in length, and provision would be required for its sweeping 

 through 300 feet of motion ; whereas with the horizontal speculum, a circular 

 building thirty feet in diameter and about sixty feet high would furnish 

 ample space, and also allow the observer, without changing his position, to 

 work entirely under shelter. 



In such an instrument the friction is reduced to a minimum by perfecting 

 the bearing of a single axis, consequently little power is required for continu- 

 ing its rotation. 



I may remark that I have used, with good effect, the regular flow of water 

 through a small turbine, in order to impart to the speculum an equal angular 

 velocity. By merely altering the velocity we are enabled to shorten or 

 lengtlien the telescope, and in a few seconds the mercury attains its equilibrium, 

 and not only the parts near the vertex are parabolic, but those also which 

 extend to the parameter, and to any distance we like to go above, leaving out 

 of consideration a very slight deviation caused by the earth's sphericity, which 

 would impart a slight tendency to the hyperbolic curve, but which, even in 

 immense instruments, would be so minute as to be within the power of 

 correction by the eye-piece of the telescope. 



It also follows that the focus can be observed by looking upwards, if the 

 vertex of the curve be removed, and those parts only used which are above its 

 parameter. 



As it is of immense importance that we should be able to concentrate a 

 large beam of light for examination of the distant nebulae, and especially for 

 spectroscopic investigations, it is not improbable that the use of such an 

 instrument, constructed on a large scale, would extend our knowledge of the 

 natural heavens, for notwithstanding all the discoveries made in the great 

 cosmic problems of creation, still, that we may be enabled to travel further 

 into what is as yet the dark profound, and to gaze with bodily eye on what 

 now form the manifold mysteries of the universe, must be the ardent wish of 

 every lover of science. 



Note. — That the above expressions are the dynamical measures of gravity 

 and centrifugal force is thus shown : — 



In circular motion the centripetal and centrifugal forces are everywhere 

 equal. Let the arc AB be described in one second ; draw BE perpendicular to 

 AS ; then in one second the body originally at A will have fallen from its 

 wonted straight path, AM, a distance = AE towards the attractive force at 



