446 Observations of « polaris for determining 
on the plate with the vapour of oxymuriate of ammonia become 
green. in the course of a few days, and that those produced by 
the vapour of phosphorus endure a very short time till they be- 
come brown with the air, and are entirely dissipated. If the 
white spot produced by the vapour of corrosive sublimate be 
uncovered, and the white crust entirely rubbed on the plate, the 
latter then unites in whitening its surface with the original 
sublimate (mercurio repr istinato) : : the same phenomenon takes 
place with the crust formed by the vapour of calomel. Hence we 
have an easy means of discovering the matter which forms a 
white crust on the surface of copper or brass plates, indepen- 
dent of arsenic. 
XCIV. Olena of a polaris, ly the Rev. Mr. L. Evans, 
of the Royal Military Academy, for determining the North 
Polar Distance of that Star at the Beginning of the Year 
1813. 
Royal Military Academy, June 16th, 1814. 
Sirs, His 5 SET but a -few days ago, seen the Astronomer 
Royal’s most excellent Catalogue of north polar distances of 84 
principal fixed stars, and noticing a curious coincidence in the 
polar distance of @ polaris, as determined with the new mural 
circle made by Mr. E. Troughton, and that of the same star 
as determined by myself with a transit circle of 24 inches dia- 
meter only, made by the same great mechanist ; I am induced 
to request your publication of the whole of my observations and 
their respective computations, with a view to encourage more 
attention to the use of transit circles of nearly the same dimen- 
sions: for 1 am inclined to believe that they ought to be ap- . 
pretiated more than they generally are. They “require, as in 
the large instruments, particular precision in their adjustments, 
prior to any observation, which ought to be taken with great 
care, calmness, and perfect ease. ° An excellent opportunity, 
too, is now afforded for comparison of observations with those 
in the Catalogue before intimated, for deciding upon their accu- 
racy. And if we take the expense of such small instruments 
ito account, we shail find, that it is not to be mentioned, when 
compared with the enormous one of the large mural or transit 
circles, which is far beyond the reach of very many ingenious 
astronomical amateurs,~of limited incomes, though they may 
have it in their power to purchase the smaller ones. Besides, 
it is my opinion, that the improvement of instruments is not ex- 
actly in the direct ratio of their magnitude. 
I am, sirs, 
Your most obedient servant, 
L, Evans. 
To Messrs. Nicholson and Tilloch. 
