On the Directive Power of Magnets, §c. 221 



be properly measured by the momentum created or destroyed in 

 that time*. 



The measure thus proposed, in fact, differs from the received 

 measure enunciated by Professor Maxwell less than might at 

 first sight be supposed; for when T is very small, (1) becomes 



work = F5 = M?;.V l . 



Thus, while I contend that the work done in a short interval of 

 time under the above circumstances is properly measured by the 

 momentum generated during the time, according to the views 

 upon the subject which are generally received it is measured by 

 the product of the momentum generated and the initial velocity 

 Vj— a position the reasonableness of which, I apprehend, it will 

 be found difficult to establish. 



5. If, instead of expressing the work done in terms of the 

 force acting and the time during which it acts, we wish to ex- 

 press it in terms of the force and of the space described under 

 its influence, we have only to replace T in the expression FT by 

 its equivalent in terms of the other variables. This, where the 

 body moves from rest, would give us \/2MFs as the measure of 

 the work done by the force F on the body M while moving- 

 through the space s. 



6 New Square, Lincoln's Inn, 

 July 29, 18/3. 



XXIV. Experiments on the Directive Power of large Steel 

 Magnets, of Bars of magnetized Soft Iron, and of Galvanic 

 Coils, in their Action on external small Magnets. By George 

 Bid dell Airy, Astronomer Royal, C.B., P.R.S. — With 

 Appendix, containing an Investigation of the Attraction of a 

 Galvanic Coil on a small Magnetic Mass. By James Stuart, 

 Esq., M.A., Fellow of Trinity College , Cambridge^. 



THE only experiments with which I am acquainted tending 

 to throw light upon the distribution of magnetic power in 

 the different parts of a steel magnet are some very imperfect 



definitely small interval of time, the former will be of a lower order of mag- 

 nitude than the latter ; while, of the small space actually described in the 

 interval, all but an indefinitely small portion will have been described under 

 the influence of the velocity from time to time generated during- the inter- 

 val ; the residuum immediately due to the direct action of the force, and in 

 no degeee resulting from acquired velocity, being of the third order of 

 small quantities at most. 



. * From this it follows that the work done by the force F acting during 

 the time T on a body which is free to move will be measured by FT. 



f From the Philosophical Transactions for 1872, Part II., having been 

 read February 8.* 



