Mr. Smeaton’s Objervation , See, ^19 
in this obfervntion, I thought it might be very pofUble for the fame 
dilappointment again to happen, with refpect to the approaching 
elongation in September. I judged, therefore, that it might 
be of fome utility to agronomy, if, by any means , a good ob* 
fervation of Mercury could be got; and alfo, that it would be 
a -proper fubjeft whereon to make trial of an inflrument for 
luch purpofes, the idea of which I had conceived, and begun 
to conftruft, above forty years before - ; but which, from va- 
rious avocations, I did not perfefl to my fatisfa&ion till the 
year 1770*: fince which time it has lain by, in hopes that 
fomething might happen, by which a full and effectual trial 
might be made thereof. 
This inflrument was orginally intended as an improvement 
of the common wire micrometer, for the purpofes of taking 
differences of right afcenfion and declination, in a more com- 
modious and effectual manner than could be done in the me- 
thod then praflifed of ufing that inflrument f ; and at the 
fame 
-for other purpofes, I refolved to ufe a double achromatic objed-glafs, made by- 
Mr. Dollond ; which being of equal aperture with the fimple objed-glafs, then 
in the tranfit of the Royal Oblervatory, this telefcope I therefore efleemed to be 
of nearly equal validity, as to quantity of light, with that at Greenwich, but 
reduced to the more commodious length of three feet fix inches. 
* Some obfervations made therewith, after it was completed, I tranfmitted to 
rny friend Mr. Aubert ; the confiftency of which induced him to procure a 
fimilar inftrument to be made for cometary and fuch kind of obfervations as cannot 
be commodioufly made in the meridian. Of this inflrument the Rev. Mr. 
Wollaston has made honourable mention in Phil. Tranf. Vol. LXXV. for 1785, 
P- 348 . 
f The common wire micrometer, as ufed by Dr. Bradley, and deferibed 
from a paper of the Dodor’s hand-writing, is given by Dr. Maskelyne in the 
Philofophical Tranfadions, Vol. LXII. for the year 1772; and in addition to 
to which I mull beg leave to obferve, that the telefcopes then in ufe for the 
Vol. LXXVII. A a a wi «-o- 
