Mr . de Mendoza Rios on 
37 2 
in order to facilitate the operation of bringing the images for 
the contact within the telescope, that author advises to make a 
preparatory memorandum of the positions which the indexes 
will nearly occupy, so that they may be set accordingly, pre- 
vious to each observation. But this method, which is always 
inconvenient, by night becomes almost impossible. For this 
reason, I have joined to the horizon index an arch LL, (Plates 
XXX. and XXXI.) which is divided, both to the right and left, 
into degrees and minutes of the Sextant, so that, when the 
glasses are parallel, the centre index coincides with the two 
first divisions o, o, and occupies the space left blank between 
them. I have further provided two sliding pieces P, P, which 
may be adapted to that arch, with a spring sufficient to keep 
them firm in any situation. Putting each of these pieces upon 
the arch, so that their ends may coincide with the divisions 
marking the rough distance to be measured, no more will be 
required, than to set the centre index alternately against each 
piece, before the beginning of the successive crossed observa- 
tion. The clamp may then be fastened, and the remainder of 
the motion produced by the adjusting screw; as, if necessary, 
the index will push the sliding piece further, and leave it at the 
point where the contact was effected. * 
The Plying Circle facilitates the use of any number of No- 
niuses, which may be applied round the whole circumference ; 
but, as the leading principle which I have chiefly had in con- 
templation, is that of obtaining an accurate result from one 
reading, I have only used a single Nonius. Two Noniuses, 
* The idea of this simple contrivance, was suggested to me by the ingenious Mr. 
E. Trouchton. 
