274 



accomplished will become apparent ; whether by wheel and pinion, 

 by a stud, by stiff friction, or by any other method. 



7. All the adjustments which are necessary in order to set the ma- 

 chine in action will be pointed out, and the order in which they 

 ought to be made will be indicated. 



8. The whole course of action of every part will be visible in 

 every stage of the progress of the machine. If it is a wheel, the time 

 and direction of its motions will appear, and also the times at which 

 it rests ; if the part is a bolt or click, the times at which it is bolted 

 or locked, and those during which it is in the reverse state, will be 

 seen. These particulars will be discovered by casting the eye down 

 the vertical line belonging to each part, which was named at the 

 top of the drawing. 



.9. In passing the eye along any of the horizontal lines, dividing 

 the cycle of the engine movements, every cotemporaneous motion, 

 as well as its direction, at that precise time becomes visible ; as also 

 the position of those parts which are at rest. 



The author then proceeds to state, that he found much time to be 

 saved in the construction of his calculating engine by employing this 

 mechanical notation ; and he has, by way of example, in this paper 

 described its application to the hydraulic ram, and to the common 

 eight-day clock, of both of which he has given the drawings and 

 notation. 



On the Parallax of the faced Stars. By J. F. W. Herschel, Esq. M.A. 

 Sec. R.S. Communicated January 19, 1826. Read March 9 and 

 16, 1826. [Phil. Trans. 1826,^.266.] 



The measurement of the distance between the two stars compo- 

 sing a double star, at opposite seasons of the year, has long ago been 

 suggested as a means of detecting a difference of parallaxes between 

 them, if any exist ; but in practice has not proved delicate enough 

 for the purpose, owing to the difficulty with which measures of di- 

 stance are still attended when minute fractions of a second are to be 

 determined. The author observes that it appears hitherto to have 

 escaped notice that a difference of the parallaxes will affect not only 

 their distance from each other, but also their angle of position, and 

 that this latter effect is much more sensible and measurable for a 

 given amount of parallax than the former ; and he accordingly pro- 

 poses it as a means of determining the question as to the existence 

 or non-existence of a sensible difference, in all stars favourably si- 

 tuated. 



He first enters into an estimate of the least amount of such diffe- 

 rence which this method is capable of detecting, with our present in- 

 struments and methods of observation, which, for two stars at 3" di- 

 stance from each other, he states to be one fortieth of a second, and 

 in closer stars a still smaller fraction. 



In selecting stars for examination, the most favourable position in 



