An Account of Mr. Lislon's improved Organ. 373 



being only an index to point out the divisions on the circle 

 within the compass : this index being set so as to make the 

 same angle with the needle, as the difference of the true 

 and magnetic meridians, will at all times give the true 

 bearings of any object observed through the two sights e 

 and f, which are diametrically opposite, and at the zero of 

 the divided circle ; at least whilst the variation of the needle 

 continues the same; but as this alters by the instrument 

 being used in a different tract of country, the brass index a 

 must be turned round a corresponding number of degrees, 

 \*hich are shown by the divided circle in the box. The re- 

 maining parts, shown in fig. 2, are AA, the upper ends of 

 the tripod supporting the instrument; B the hall and socket 

 by which the box is adjusted, so as to be horizontal ; the 

 stem of the ball has a circle c fixed to it, carrying two 

 sights g- A, through which the observations are taken; the 

 circle c is divided into degrees, and the compass-box, which 

 turns round upon it, has a vernier applying to them : this 

 renders the instrument a theodolite, as any two objects may 

 be observed through the sights ef, and g k, and the divi- 

 sions on the circle c will denote the angle between them, 

 and at the same time the bearing of any object may be 

 taken by observing it through the sights ef, and noting 

 the degree pointed out by the brass needle a, which is the 

 true bearing required, without any allowance for the varia- 

 tion, as was the case in the common instrument. 



I-V. On the Rev. Mr. Liston's Euharmonic Organ, 

 and his *' Essay on perfect Inionatmi," just published^ 

 for explaining fully the Principles of Tuning, and of per- 

 forming upon thisOrgan with peifect Harmonies, in almost 

 every possible Fariety of Keys and Passages. By Mr. 

 John Farey Sen. 



To Mr. Tilloch. 



StR, X HE work which has for near a year past, been an- 

 xiously exptcted by great numbers, for explaining the prin- 

 ciples and practice of a Musical Scale, without any tem- 

 perament or imperfections ia its harmony, has at length ap- 

 peared and is to be had of Messrs. Longman and Rees, or 

 any other Bookseller, or at the Music- sellers, and I take 

 the earliest opportunity, of mentioning a few particulars 

 therefrom, in addition to what I communicated in your 



37 th 



