TRANSACTIONS OF THE SECTIONS. 4?, 



fore-and-aft line of the sliip ; the centres of the compasses must be exactly in a 

 line at right angles to the head of the ship ; the adjusting magnets must follow the 

 same accuracy of an-angement ; and we are thus day by day approaching to the 

 necessity of being forced to expend on an instrument, so common, but so valuable, 

 and which imfortimately seamen in general, and I may venture to add, iron ship- 

 builders in particular, often ti-eat so lightly, the same rigid accuracy of fittings and 

 attention that are requh-ed in the more delicate instnmients of the observatoiy. I 

 may add that ^vithul the last few days I have been iuformed that the steering 

 compasses of ' La Glou-e' (the ' Warrior ' of France) are placed within a similar rifle- 

 tower on the upper deck. I am further infonned by M. Darondeau, who holds 

 with respect to the French Imperial Navj' a somewhat analogous position to my 

 own in the Royal Navy of this coimtiy, that this is an unadvisable arrangement. 

 ' In concluding these brief remarks, I cannot but convey to the Meeting the deep 

 debt that the seamen of all nations owe to the President for his long and patient 

 investigation on theu' behalf, of the management of the mariners compass imder 

 difficulties; to him we are indebted for the firet practical rules on the subject; and 

 however opinions may have varied -as to the uses of con-ecting magnets imder the 

 old condition of things, there can be no. doubt that in, the case of the '.Warrior's,' 

 main-deck compass, this system' in its main features becomes an absolute necessity-.' 



Qn the Photographic Records (jlven at the Keiv Observatory of the great Mag- 

 ■' netic Storm of the end of August and beginning of September 1859. By. 



B. Stewaet, A.m. 



The author remarked that the tendency of this great magnetic storm was to 

 decrease the horizontal and vertical components of the earth's force, and that the 

 disturbing force came in a wave, the period of which was seven hours. He con- 

 trasted this lengthened period with that of earth-currents, which is only a few 

 minutes, and supposed that the change in the earth's magnetism is due to the abso- 

 lute amount of a disturbing force, which is of a fluctuating character, and of which 

 the fluctuations produce the earth- ciuTents and Aurora JBorealis, which are thus 

 regarded as secondary discharges. 



On the Amount of the direct Magnetic Effect of the Sun or Moon on Instruments 

 at the Earth's Surface. By G. JoiorsToifE SioifET, M.A., F.R.S. 



In the Philosophical Magazine for March 1858, Dr. Lloyd showed that the 

 observed disturbances of the magnetic needle, depending on the hours of lunar and 

 solar time, follow laws inconsistent with their being due to the direct magnetic attrac- 

 tion of the moon or sim. Hence it might be too hastily concluded, from the absence 

 of observed eff'ects following the proper laws, that these luminai-ies are not magnetic. 

 The design of Mr. Stoney's commimication was to show that, though as highly mag- 

 netized as the eaith, then- direct eflects wovdd be almost inappreciable. 



The maximum moment which the moon could impress on the needle was first 



* AT M 



ascertained to be 2 ^^yvT' '^l^^^® ^^ ^^^ ^^' ^'^ the magnetic moments of the moon 



and needle, and D the interval between their centi-es. It follows from this that we 

 may substitute for the moon a globe a meti-e in diameter of equally magnetized 

 materials, and placed at such a distance as to subtend at the needle an angle equal 

 to the greatest apparent diameter of the moon as seen from the earth's surface. By 

 applying to the problem in this foi-m the wonderful numerical data elicited from 

 the observations by the genius of Gauss in his memoir on the magnetism of the 

 eai'th, the gi-eatest d'rect disturbance which the moon could produce, on the liypo- 

 thesis of its being of materials as magnetic bulk for bulk as the earth, proves to be 

 less than a tenth of a second of space on the declination-needle, and less than a 

 twenty-seventh on the dipping-needle. 



The observations with wliich these should be compared have been made at 

 several stations. The principal part of the observed lunar-diurnal variation con- 

 sists of a tenn depending on twice the lunar hom'-angle, but there is also a small term 

 containing the simple hour-angle. This latter is the one which, as Dr. Lloyd ha^ 



