2 PROFESSOR FORBES'S EXPERIMENTS ON 



sity of the earth's magnetism at different places, or at the same place at different 

 times, will be (on the principle of the pendulum) inversely as the square of the 

 time required to perform a given number of vibrations in infinitely small arcs, 

 under the different circumstances. But various adjustments have to be attended 

 to, and corrections applied. 



5. Nothing more portable or more simple than the instrument in its pre- 

 sent form can be desired. These requisites are no doubt obtained at the expense 

 of some accuracy. Mr Snow Harris has shewn * that the influence of the sur- 

 rounding air upon the needle gives rise to considerable errors, especially when 

 the needle is so small and light, as in Hansteen's apparatus. But greatly to en- 

 large the needle, and to connect an air-pump with the apparatus, is nearly equi- 

 valent to depriving the traveller of its use altogether. Hansteen's instrument 

 was the constant companion of my pedestrian excursions, and had it been in any 

 other form, the present observations would probably never have been made. Be- 

 sides all this, there are sources of error arising from imperfectly lmo^^^l and irre- 

 gular variations of the earth's intensity, and equally or more important ones from 

 changes of magnetic intensity in the needle itself, which the improvements in 

 question do not affect. Until by a regular and long continued series of observa- 

 tions, such as those likely to be undertaken at Greenwich, magnetism shaU be re- 

 duced to more of a science than it is at present, we must beware of pretending to 

 illusory accuracy in a traveller's detached experiments. Those about to be de- 

 tailed in this paper, will sufficiently indicate the degree of comparability of obser- 

 vations made with Hansteen's instrument, such as it is, and which is by far the 

 best test of their real value. It has certainly rather exceeded than fallen short of 

 my expectations. 



^1. Adjustments and Method of ohserving. 



6. Hansteen's instrument contains no provision for securing the horizonta- 

 lity of the needle, which is of considerable importance. The needles have, indeed, 

 sliding collars of suspension, which may be altered with change of dip, but the 

 box has no adjusting levels. I have always f used a small spirit level for adjust- 

 ing the bottom of the box, and then, as carefully as I could, made the needle hang 

 parallel to it, but the adjustment was troublesome and unsatisfactoiy. 



7. The needle being levelled and allowed to come to rest, it was drawn out 

 of its position of rest ±, but always in a horizontal plane, by the approach of a 



* Edinburgh Transactions, vol. xii. p. 1. — See also the Observations of Professor Bache ; Ame- 

 rican Phil. Trans, vol. v. 



t I cannot answer, however, for two or three of the first observations hereafter to be quoted. 



X As the torsion of the silk fibre must have some influence, it is not unimportant to remark, that 

 the same thread which was adapted to. the instrument in August 1832, has been used ever since. 



