TRANSACTIONS OF THE SECTIONS. 29- 



downwards a considerable depth into the body of the moon. In short, we have no 

 likeness to it on earth, in the sense now spoken of, except our great trap and crystalline 

 dykes. It seemed clear, then, that the intersection of these rays are really intersections 

 of dislocations, from which we might deduce their chronology. Can the intersection, 

 however, be suflSciently seen? in other words. Is the telescope adequate to deter- 

 mine which of the two intersecting lines has disturbed or cut through the other? Prof. 

 Nichol maintained the affirmative in many cases, and by aid of diagrams, taken down 

 from direct observation, illustrated and enforced his views. 



Note on Solar Refraction. By Professor C. PiAzzi Smyth, JF?oyaZ Observa- 

 tort/, EdinbuTffh. 



Amongst other interesting and important consequences of the dynamical theory 

 of heat. Prof. W. Thomson having deduced the necessity of a resisting medium, 

 the condensation of this about the sun, and a consequent refraction of the stars 

 seen in that neighbourhood. Prof. Piazzi Smyth had endeavoured to ascertain by 

 direct astronomical observation, whether any such effect were sensible to our best 

 astronomical instruments. Owing to atmospheric disturbances, only three ob- 

 servations, yielding two results, had been yet obtained ; but both these indicated a 

 sensible amount of solar refraction. Should this effect be confirmed by more 

 numerous observations, it must have important bearings on every branch of astro- 

 nomy ; and as the atmosphere at all ordinary observatories presents almost 

 insuperable obstacles, the author pointed out the advantage of stationing a tele- 

 scope for this purpose on the summit of a high mountain. 



On Altitude Observations at Sea. By Professor C. Piazzi Smyth, 

 Edinburgh. 



This paper treated mainly of the observation of altitudes at sea under circum- 

 stances when at present they are generally unattainable, viz. when the sea horizon 

 is not visible. After a statement of the necessary principles which should guide 

 inventors in this matter, the author exhibited a new species of artificial horizon, 

 which allowed all the latitude of the natural horizon as to errors in the position of 

 the whole sextant ; and while exhibiting extreme sensibility to angular movement, 

 was very little affected by any horizontal disturbance or translation through space. 



Any still outstanding difficulties were effectually removed by the employment, in 

 addition, of a stand, which taking advantage of ttie composition of rotatory motion 

 and the permanence of an axis of rotation, as seen on the grand scale in the con- 

 stancy of the annual direction of the earth's pole, or in the pheenomenon of the 

 precession of the equinoxes, and in a small way in a spinning-top, completely 

 eliminated all the angular movements of which a ship is capable. 



This second subject of the paper concludes thus : — " To the first idea of taking 

 advantage of the general principle for the present purpose, I believe that I was led in 

 a great degree by the eminently clear and practical manner in which the Rev. Baden 

 Powell exhibited by models, and expounded in his lectures in 1852 and 1853, the 

 action of the composition of rotatory motion under various circumstances in nature 

 and in art; for then I perceived why ' Troughton's top' had so narrowly, but 

 completely escaped the honour of becoming a useful instrument to nautical astro- 

 nomy; and how what was good in it might be transferred to a better planned 

 apparatus. More recently, as every one knows, M. Foucault has added a degree of 

 glory even to the mechanical law, by employing another feature of it in his ' gyro- 

 scope,' as a means of detecting and exhibiting the rotation of the earth." 



On the Transmission of Time Signals. By Professor C. Piazzi Smyth. 

 After alluding to the general subject of the longitude, the very large number of 

 ships lost during the past year through errors of their longitude, and the recognized 

 aids that have been furnished to seamen in the erection of time-balls, the author 



