253 



I cannot but suspect that the performance of this telescope is af- 

 fected by temperature, and that severe tests in the summer months 

 might afford different conclusions to those which I have arrived at • but 

 as I considered my opinion was desired on the instrument in its present 

 state, I took no means for applying artificial heat. And, perhaps, the 

 secondary spectrum which haunts the field might be mitigated, and the 

 prismatic colours destroyed, by an alteration of the distance between 

 the fluid and outer lenses ; but the same consideration prevented my 

 applying for a screw, by which it might have been effected. 



But there is one condition of the instrument which, if correct, would 

 be of greater importance than the rest, as connected with this Report, 

 It strikes me forcibly, from the several effects I observed, that the focus 

 has been cut too short ; a defect which would seriously affect the 

 spherical aberration of the outer or object lens and its dispersion : 

 and this would account for the fluid refractor not performing better 

 than the flint-glass one, without impugning the corrective powers of 

 the sulphuret of carbon, or its skilful application by the scientific 

 Professor. 



April 4, 1833. W. H. Smyth. 



A paper was then read, entitled, " An Account of some Experi- 

 ments made in the West Indies and North America, to determine 

 the relative Magnetic Forces, in the years 1831, 32, and 33." Bv the 

 Rev. George Fisher, M.A., F.R.S. 



The experiments of wmich the results are given in this paper were 

 made by Mr. James Napier, late Master of H. M. S. Winchester. 

 The needles were precisely similar to those used in the experiments 

 described by the author in a former paper; and the observations were 

 made with great care, and repeated several times at the same places ■ 

 by which it appeared that the intensities of the needles continued 

 unchanged during the whole period of the experiments; and the 

 mean of all those made at one place was taken as the result. From 

 these the relative forces at different places were computed, and stated 

 in the form of a table. 



A paper was also read, entitled, " On the Theory of the Moon." 

 By John William Lubbock, Esq., V.P. and Treas. R.JS. 



M. Poisson, in a memoir which he has lately published on theTheory 

 of the Moon, expresses the three coordinates of her path, namely, 

 her true longitude, her distances, and her true latitude, in terms of 

 the time. The author observes that the reasons for so doing adduced 

 by M. Poisson, are the same as those which led Mr. Lubbock also to 

 deviate from the course which had previously been always pursued 

 by mathematicians, and to employ equations in which the true lon- 

 gitude is the independent variable. Instead, however, of integrating 

 the equations of motion by the method of indeterminate coefficients, 

 as the author had proposed, M. Poisson recommends the adoption of 

 the method of the variation of the elliptic constants. In the present 

 paper, Mr. Lubbock states the reasons which have determined him 

 not to employ the latter method, founded chiefly on the advantages 

 of obtaining complete uniformity in the methods used in the theories 



