Royal Astronomical Society. 71 



magnetic force passing through the metals is the same in all the 

 cases, and hence the polarity within them the same. 



The author then gives a more explicit meaning, in accordance 

 with the definition of line of magnetic force contained in this paper, 

 to some of the expressions used in the three last series of his 

 Researches on Magnetic Condition, Atmospheric Magnetism, &c. : 

 and by referring to former results obtained since the year 1830, 

 illustrates how much the idea of lines of force has influenced the 

 course of his investigations, and the results obtained at different 

 times, and the extent to which he has been indebted to it ; and then, 

 recommending for many special reasons the mode of examining 

 magnetic forces by the aid of a moving conductor, he brings for the 

 present his subject to a conclusion. 



ROYAL ASTRONOMICAL SOCIETY. 

 [Continued from vol. h. p. 326.] 



June 13, 1851. — On some Improvements in Reflecting Instru- 

 ments. By Prof. Piazzi Smyth. 



In the course of his lectures on Practical Astronomy to the 

 students of the Edinburgh University last winter, Prof. P. Smyth 

 had unusual opportunity of ascertaining those points in the making 

 of the generality of the observations of navigators by sea and of 

 travellers on land, which presented the greatest difficulty to be- 

 ginners. And as these points generally consisted of needless pe- 

 culiarities, sometimes absolute imperfections in the instruments, 

 the Professor proceeded to remove them as well as he could, and 

 the result may, perhaps, be more extensively useful, especially as 

 the difficulties were generally felt on the sextant being applied to 

 observations of stars by night, a more exact means than the sun 

 by day, and therefore to be encouraged and assisted in every way. 



In naval observations the impediments were, both by the expe- 

 rience of the class, and by the testimony of naval officers, — 



1st. Difficulty of seeing and of bringing down the star. 



2nd. Difficulty of seeing the horizon line at night. 



3rd. Difficulty of reading off the angle on the limb. 



The first of these, in so far as it depended on the dark field of 

 the telescope, he proposed to remedy by employing a telescope of 

 large aperture, say 2 inches, in place of the usual size, \ or 

 5 inch ; in so far as the loss of light was occasioned by reflexion and 

 absorption at the glasses, he intended to remove this by employing 

 metal reflectors, by which, too, the occasional nuisance of second 

 imageg would be avoided and greater accuracy obtained. He had 

 tried speculum metal for the purpose with great advantage; but, 

 under some circumstances, he was in hopes of being able to employ 

 silver, which has lately been found to be capable of reflecting near 

 double the amount of light that speculum metal does, though that 

 more than quicksilvered glass ; and then, in so far as the 

 loss of the star in "bringing clown" is caused by the diminished 

 surface exposed by the index-glass at large angles, he proposed to 

 make that larger than usual : besides which, the reflexion taking 



