THE MAKERSTOUN OBSERVATIONS. H 



till each length of fibre bore nearly an equal strain. Third, That the thread 

 should be as small as is consistent with durability. The number of fibres for 

 a particular weight should be determined by experiment ; sixteen fibres of the 

 silk supplied to the Makerstoun Observatory, and bearing nearly a pound weight, 

 were found insufficient several times, though that number was recommended in 

 the Report of the Royal Society ; this was probably due to some difference in the 

 thread : a thread of 22 fibres has now performed well for seven years. 



I should notice that M. Nervander has proposed to form the suspension 

 thread, by moistening with hot water the fibre cut into lengths, and submitting 

 each length in this state to a considerable tension, before combining them to form 

 the thread. 



The Bifilar Magnetometer. — The chief source of error for this instrument is 

 also to be found in its suspension ; wires of silver or of gold have generally been 

 adopted ; although threads like those for the declinometer have also been employed, 

 as at Greenwich. I object to the use of skeins, not merely because the errors 

 due to a silk suspension are probably greater than those due to wires, but chiefly 

 because no correction can be applied for the errors due to the former, while those 

 due to the latter can be wholly eliminated. One error common to every kind of 

 suspension is due to the stretching of the thread or wires, which gives a false 

 value of the secular change ; it is curious that no attempt has been made to 

 eliminate this error, which could easily be done by means of a small apparatus 

 for measuring the distance between the suspended magnet and the base plate of 

 instrument. Any twist in the wires will give a false value of the unit coefficient, 

 if determined in the usual way by the torsion circle ; I am afraid that there is an 

 equal chance of a similar error with silk threads, hesides the probable variation 

 due to breaking of fibres, &c. The error arising from twist, however, does not 

 appear, when the process, which I have proposed and used, of determining the 

 unit coefficient by deflections is adopted. The error which is peculiar to the wire 

 suspension is that already noticed of a variation due to temperature ; this error, 

 however, also disappears when the temperature coefficient is determined by my 

 process, since then, the total effect of temperature upon the position of the magnet 

 is at once obtained. There remains against the silk suspension the heavy and in- 

 determinable errors due to humidity, and to the gradual breaking of fibres. 



By the use of metallic wires and an apparatus to determine the amount which 

 they stretch, nothing is required to render this instrument as perfect as possible 

 but a magnet with permanent magnetism, the proper processes being employed 

 for the determinations of the unit and temperature coefficients. 



The Balance Magnetometer.- — I conceive that with proper care this apparatus 

 may give considerably better results than have ever yet been obtained from it. 



