160 Intelligence and Miscellaneous Articles. 



equal and opposite in direction, which is required to hold the machine 

 fixed in its position of equilibrium. A lever-arm is fixed to the 

 machine or cradle in a horizontal position and provided with a sliding 

 weight of known value, sufficient to hold the machine fixed in its 

 position of equilibrium when in the performance of its duty. The 

 couple required can thus be known in terms of lever-arm and weight. 

 We then only need to know the number of revolutions in a unit of 

 time, when we have all the data needed in order to compute the 

 energy. 



If W denote the weight, L the lever-arm, and n the number of 

 revolutions in a minute, we shall have : Energy =2 7rWL n, as in 

 the case of the well-known Prony brake. 



For purposes of accurate scientific inquiry, the field-magnets alone 

 may be mounted and balanced on knife-edges, so as to turn freely 

 like the beam of a common balance. By this plan all useless work 

 is excluded from the account. Friction at the bearings and at the 

 brushes do not in this case have any tendency to make the field- 

 magnets revolve. 



In the Pnysical Laboratory of this institution there are several 

 machines having the construction here pointed out. They leave 

 nothing to be desired in point of sensitiveness or accuracy in their 

 indications. — Silliman's American Journal, January 1884. 



Princeton, Nov. 23, 1884. 



THE ERGOMETER. 

 To the Editors of the Philosophical Magazine and Journal. 



132 Marlborough Road, Cheetham, 

 Gentlemen, Manchester, January 21, 1884. 



In reference to the article on the Ergometer, by Mr. Frederick 

 John Smith, B.A., in your last number of the Philosophical Maga- 

 zine, wherein he describes an arrangement devised by himself for 

 overcoming some frictional difficulties in the integrating apparatus 

 of Ashton and Storey's Power Meter, I beg to state that I devised 

 precisely the same arrangement seven or eight years ago, but the 

 rapid wear and tear of the small friction-disks or rollers rendered 

 the cure worse than the disorder, so of course I never brought it 

 into notice. I have, however, patented, and shall shortly have 

 before the public, an integrating arrangement having a rolling con- 

 tact with a positive and not a frictional connexion, whereby a 

 positive motion is given to the recording indices. 



I am, Gentlemen, 



Yours faithfully, 



Wm. Ashton, 



