8 REPORT — 1842. 



35 minutes of arc, occasionally takes place in the time required for the three 

 vibrations which give the mean place entered in the tables. . 



Experience has somewhat diminished the value of the term observations as 

 a principal means of detecting disturbances ; especially since our observatories 

 have adopted hourly observations, by which a departure from the normal 

 state cannot continue long without notice ; and thus the disturbance furnishes, 

 of itself, at all the stations, a natural signal for extra and simultaneous ob- 

 servation. Besides our Colonial and East India observatories, the disturbances 

 are watched with the greatest diligence at Prague, Munich, and Greenwich. 



6. New Magnetic Instruments and Modes of Observation. 

 Transportable Magnetometer. — Among the most useful services that have 

 been rendered to the magnetic cause in the year elapsed, has been the ma- 

 king this a thoroughly practical instrument, and the instructions which have 

 been drawn up by Mr. Riddell for its use. These instructions are now in 

 progress of printing. They are of the most distinct and practical kind, and 

 contain the formulae of correction and reduction with numerical examples 

 from actual practice. This instrument and the instructions for its use, freely 

 supplied to officers of the army and navy, will multiply absolute determina- 

 tions, teriu observations, and disturbance observations far beyond what could 

 ever be done by fixed observatories, and also in localities where such could 

 not be established. Already have these improved instruments been supplied 

 to the ships of Capt. Blackwood and Capt. Sulivan, the first proceeding to 

 survey Torres Strait, and the latter the Falkland Islands, and the officers have 

 been instructed in the use of the instruments at Woolwich. Another has 

 been supplied to Mr. Lefroy, and will enable him to keep the terms in what- 

 ever part of America he may find himself on the days appointed. Others are 

 preparing for Capt. Barnett and Capt. Grove, R.N., who are engaged in sur- 

 veys at Bermuda and Malta, both being stationary for several months in the 

 winter. From Capt. William Allen, R.N., of the Niger expedition, we have 

 received the November and December terms, 1841, kept at Ascension, with 

 the original German transportable magnetometer, as also the absolute deter- 

 minations made with it in the same island. 



Professor Lloyd's new Inclinometer. — The vertical element of the magnetic 

 force is one of difficult determination with that precision which is required to 

 become comparable with the determinations of its horizontal component, so as 

 to afford a perfectly correct means of calculating the changes of the incli- 

 nation and of the total intensity. The instrument hitherto used for the 

 purpose, called by Prof. Lloyd the Balance Magnetometer, has been found 

 scarcely adequate to its intended purpose. " Unexceptionable as its principle 

 is in theory, the accuracy of its results has not been commensurate with that 

 of the others. This inferiority is owing to the large influence which the un- 

 avoidable errors of workmanship necessarily have on the position of equili- 

 brium of a magnet supported on a fixed axle." " The sources of error seem 

 to be inherent in every direct process for determining the third element, and 

 it is only by an indirect method that we can hope to evade them."* Such a 

 method has accordingly been recently proposed and subjected to trial by Dr. 

 Lloyd, by means of an instrument to which he has given the name of an 

 Induction Inclinometer, the principle of which is the measurement of the 

 intensity of the magnetism induced on a vertical bar of soft iron (which must be 

 considered as due to the vertical magnetic component only) by the deviation it 

 is capable of causing in a horizontal bar suspended near it. The details of 



* Account of the Magnetical Observatory at Dublin, &c, by the Rev. H. Lloyd, D.D. 



