20 Mr. Smeaton’s Ohfervations on the 
threads of this fcrew were ' not formed upon a cylindrlc fur- 
face, but upon a folid whofe fides were terminated by arches 
of circles. The whole length contained fifteen threads ; and 
as every thread (on the fide next the wheel) pointed towards 
the center thereof, the whole fifteen were in contadt together ; 
and had been fo ground with the wheel, that, to my great 
aftonifhment, I found the fcrew would turn round with the 
utmoft freedom, interlocked with the teeth of the wheel, and 
Would draw the wheel round withput any fhake or flicking, or 
the lead fenfation of inequality. 
How long this engine might have been made before this firfl 
interview, I cannot now exactly afcertain : 1 believe not more 
than about a couple of years ; but this I well remember, that he 
then fhewed me an inflrument intended for aflronomical pur- 
pofes, which mu ft have been produced from the engine, and 
which of itfelf mufl have taken fome time in making 
I in 
*. This inflrument was of the equatorial kind ; the wheel parallel to the 
equator, the quadrant of latitude, and femi-circle of declination, being all 
furnilhed with ferews containing fifteen threads each, framed and moved in the 
fame manner as that of the engine; the whole of which inflrument was already 
framed, and the telefcope tube in its place, which was intended to be of the 
inverting refra£ting kind, and to be furnilhed with a micrometer. This, however, 
was not compleated till fome years after; but, fin the year 1748, I received it m. 
London for fale. It ftaid with me two years, in which time I fhewed it to all my 
mechanical and philofophical friends, amongfl whom was Mr. Short, who after- 
wards publifhed in the Philofbphical Tranfa&ions, vol. XLYI. N° 493* p. 241. an 
Account of a portable Obfervatory, but without claiming any particular merit 
from the contrivance. However, the model of it differs from Hindley’s equatorial 
only in the following articles. He added an azimuth circle and compafs at the 
bottom. He omitted the endlefs ferews, placing verniers in their Head ; and at 
the top, a refledting telefcope inflead of a refrabtor. This inflrument of Hind- 
•iey’s being afterwards returned to him unfold, I pointed out the principal deficien- 
cies that I found therein ; viz 0 that, in putting the inflrument into different pofi- 
lions 
