2 1 6 Royal Society. 



dip j so that, by this means, the observations usually required for that 

 purpose, and which are of the most tedious nature, will be avoided. 

 To effect both these objects in the most convenient manner, he pro- 

 poses that the needle should be so constructed that its centre of gra- 

 vity should be out of the axis of motion, in a line perpendicular to that 

 axis and to the axis of the needle. The requisite formulae for deter- 

 mining the dip and the measure of the terrestrial intensity, in this 

 case and also when the centre of gravity is in any other position, are 

 investigated in the paper. Mayer had previously pointed out that the 

 dip might be determined by means of a needle having its centre of 

 gravity out of the axis of motion, and had given the formulae requisite 

 for that purpose. His object, however, does not appear to have been 

 the same as our author's, — the avoiding in all cases that source of 

 inaccuracy, the inversion of the poles of the needle,— but simply the 

 determination of the dip, whether the centre of gravity of the needle 

 were made to coincide with the centre of motion, or not : the deter- 

 mination of a measure of the terrestrial intensity, by such means, does 

 not appear to have entered into his contemplation. 



As another form in which the same principles might be advan- 

 tageously applied, the author proposes that two needles, similar in all 

 respects, should be placed on the same axis; and points out how, by 

 means of such a compound needle, both the dip and intensity might 

 be determined by independent methods, so that the agreement of the 

 results would afford a test of the accuracy of the adjustments and of 

 the observations. He considers that the knife-edge support, which 

 has recently been adapted to a dipping needle, would be peculiarly 

 applicable to a needle of this construction. The sensibility of such a 

 needle would be much greater than that of any hitherto constructed, 

 and the utmost delicacy would be required in the adjustments ; but 

 if the needle were accurately constructed, and due care were taken in 

 the magnetizing, and in making the adjustments and observations, 

 the author expects that the dip and intensity would be determined to 

 a degree of certainty hitherto unattained. 



The advantages proposed to be derived from the use of a dipping 

 needle on the principle described in this paper, are, that as the dip 

 would be obtained without inversion of the poles, the results would 

 be less liable to error than when that operation is necessary, and the 

 observations would be made in less than half the time usually re- 

 quired; and that a measure of the intensity of terrestrial magnetism 

 would be obtained from the same observations which give the dip, 

 the intensity of the force being thus always determined by means of 

 the same needle, and at the same instant as its direction. 



May 2. — A paper was read, entitled, " Essay towards a first ap- 

 proximation to a Map of Cotidal Lines." By the Rev. William 

 Whewell, M.A.F.R.S. Fellow of Trinity College Cambridge. 



The general explanation of the phenomena of the tides originally 

 given by Newton, although assented to by all subsequent philoso- 

 phers, has never been pursued in all the details of which its results 

 are susceptible, so as to show its bearing on the more special and 

 local phenomena, to connect the actual tides of all the different parts 



