558 PHILOSOPHICAL TRANSACTIONS. [anNO 1755. 



ing the state of the variation, is now at hand, without which the above-men- 

 tioned valuable advantages of the chart will be lost to the mariner ; they have 

 determined to collect and compare all the observations that can be procured by 

 them, in the space of a year from this time, or so long after as the return of the 

 East-India ships then next following ; if such delay should become necessary, by 

 the arising of any doubt in consequence of such comparison ; and then to publish 

 the result of their process, in such a manner as shall seem most convenient. 



Several of the learned and ingenious have endeavoured to account for this 

 phenomenon of the variation of the magnetic needle, and its continual mutation ; 

 whence different methods of computation have been proposed, by which they 

 have endeavoured to determine what the quantity of the variation, according tq 

 their several hypotheses, will be at any given place and time: the above propo- 

 sition therefore will, if carried into execution, bring these severally to the test, 

 and enable the judicious either to approve or reject them ; the writers being de- 

 termined to publish nothing which shall not be warranted by the real observa- 

 tions, which shall come into their hands, and shall leave the application to others: 

 if any of them should be so far confirmed, by this examination and comparison, 

 as to give just ground for a calculation, their labour will be at an end ; but if 

 not, they humbly recommend the continuance of such a periodic operation, as 

 they now propose to undertake, being the only means of attaining such a desirable 

 event, and of supplying the defect till it can be obtained. 



END OF THE PORTY-EIGHTH VOLUME OF THE ORIGINAL. 



Art. I. On the Pressure of Weights on Moving Machines. By Christian Hee, 

 Professor of Mathematics and Experimental Philosophy in the Marine Insti- 

 tution of Copenhagen. From the Latin. Vol. XLIX. p. \. 



Let fig. 9, pi. 11, represent an axe-in-peritrochio. Let a be the moving power ; 

 its distance fi-om the centre of motion a ; also b the weight, h its distance from 

 the centre ; and c the radius of the axis where the friction is. Further, let m 

 denote the weight of the machine, and d the distance of the centre of the forces 

 from the centre of gravity. It is required to find the pressure on the axis, when 

 the descending power a actuates the machine. 



If now the pressure arising from the descending power a, or that by which 

 the thread is stretched at the side a, be called %; then, by the equality of action 

 and re-action, the pressure or tension at the other side j3 will be = -7 ; hence the 

 whole pressure, exclusive of the weight of the machine and cord, will be = tt + 



