345 



the two needles, it appears that at the time when an increase took 

 place in the intensity of the dipping-needle, that of the horizontal- 

 needle underwent a corresponding diminution, and vice versd. On 

 comparing these results with the hypothesis of a rotation of the ge- 

 neral polarizing axis of the earth about its mean position as a centre, 

 and employing for this investigation the formulae given by Mr. Bar- 

 low, in his Essay on Magnetic Attractions, it is found that the radius 

 of this circle of rotation is very nearly eight minutes. The magni- 

 tude of this radius, however, "will be considerably influenced by the 

 sun's declination. 



The change of intensity of the dipping-needle, in as far as it is 

 owing to a variation of the dip, would be only in the proportion of 

 3726 to 3732 ; whereas its actual amount is found to be one eighty- 

 third part of the whole. This, therefore, seems to imply changes in 

 the general magnetic intensity of the earth. But the author, limit- 

 ing his present inquiry to the variations in the dip, concludes that 

 the times of the day when these changes are the greatest and the 

 least, are such as indicate a constant inflexion of the magnetic pole 

 towards the sun during the diurnal rotation, and to point to the sun 

 as the primary agent in the production of these changes. 



Experiments relative to the Effect of Temperature on the refractive 

 Index and dispersive Power of Expansible Fluids, and on the In- 

 fluence of these Changes in a Telescope with a fluid Lens. By Peter 

 Barlow, Esq. F.R.S. %c. Read May 15, 1828. [Phil Trans. 

 1828, p. 313.] 



In a paper lately read to the Society, the author stated that he 

 had not perceived any change in the focal length of the telescope, 

 induced by changes of temperature ; but he has since ascertained 

 that in order to produce the brightest and most perfect image, the 

 distance of the object-glass requires a minute adjustment, amounting 

 to 0'134 of an inch, corresponding to an elevation of temperature 

 from 57 to 84, or a depression from 57 to 31. 



In order to introduce greater clearness and precision, the author 

 proceeds to define certain terms which he finds it necessary to em- 

 ploy. By the length of the telescope, he would be understood to mean 

 the distance between the object-glass and the focus ; by the fluid 

 focus, that between the fluid lens and the focus ; and by the focal 

 power of the telescope, he means the focal length of a telescope of 

 the usual construction, which gives the same convergency to the 

 rays, or produces an image of the same size : but he also employs the 

 term focal length of the telescope, as synonymous with the first ; that 

 of fluid focal length as synonymous with the second ; and that of 

 equivalent focal length as synonymous with the last of these terms. 



As it is difficult to determine the refractive index of the fluid under 

 different circumstances, from which their effects on the focal power 

 of the telescope might be computed, the author endeavoured to as- 

 certain, by direct observations, . the effect of changes of temperature 



