290 Afr. CavAllo’s Defcription of 
minutes, &c. What has been faid of minutes may he faid of 
ieconds alfb, when the fcale is to be applied to a large 
telefcope. 
Thus far this micrometer, and its general ufe, have been 
fufficiently described, and mathematical perfons may eafily 
apply it to the various purpofes to which micrometers have 
been found fubfervient. But as the fimplicity, cheapnefs, and 
at the fame time the accuracy of this contrivance, may render 
the ufe of it much more general than that of any other mi- 
crometer ; and I may venture to fay, that it will be found 
very ufeful in the army, and amongft fea-faring people, for the 
determination of diftances, heights, &c. ; I (hall therefore fub- 
join forne practical rules to render this micrometer ufoful toper- 
fens unacquainted with trigonometry and the ufe of logarithms. 
Problem I. The angle, not exceeding one degree, which 
is fubtended by an extenfion of one foot being given, to find 
its diftance from the place of obfervation. 
N. B* This extenfion of one foot, or any other which may 
be mentioned hereafter, muft be perpendicular to the direftion 
of the telefcope through which it is obferved. The diftances 
are reckoned from the obje£l-glafs of the telefcope, and the 
anfwers obtained by the rules of this problem, though not 
exa£Uy true, are however fo little different from the truth, 
that the difference feluom amounts to more than two or three 
inches, which may be fafely negleftedv 
Hule 1. If the angle be expreffed in minutes, fay, as the 
given angle is to 60, fo is 68 7,55 to a fourth proportional, 
which gives the anfwer in inches. 
2. If the angle be expreffed in feconds, fay, as the given 
angle is to 3600, fo is 687,55 to a fourth proportional, which 
expreffes the anfwer in inches. 
7 3* If 
