the Trigonometrical Operation . 149 
tions of the head, are the number of minutes, always lefs than 
15', to be added. If there be no odd feconds, the dart will then 
ftand at 6o° on the head ; but, if any number of feconds are to- 
be added, the dart will (hew, by its pofition with regard to 6o°, 
what that number is. Thus, by adding the parts together, the 
meafure of the total angle is obtained. 
The conftrudtion, adjuftment, and application of thefe ver- 
tical microfcopes have been given more fully, becaufe they form 
a mofteffential part of the inftrument : for the fixed wire con- 
flan tly remaining on its dot, the fradlional fpace may be repeat- 
edly meafured many times over, if neceflary, anda mean refult 
may then be taken. But it rarely happens that two observers, 
reading off with the oppolite microfcopes, differ more than half a 
fecond from each other at the very firft reading. If time 
therefore permits, and the circumftances of the weather fhould 
alfo be favourable for repeating the obfervation with the tele-* 
fcope, it is fufficiently obvious to what a wonderful degree of 
accuracy the meafure of angles may in this way be obtained. 
Art. XV. Horizontal Microfcopes . 
Befides the two vertical microfcopes, applied in the manner 
that has been defcribed to the meafurement of the fradlional 
fpace in horizontal angles, there is yet another to be men- 
tioned, which is placed horizontally on the har that carries the 
tranfit telefcope, and is directed to the divifions on the femi- 
circle attached to its axis, for the meafurement of angles of 
elevation or depreffion, as has already been taken notice of 
This microfcope, which is of the fame conftrudlion with the 
others, but larger, being upwards of nine inches in length, is 
reprefented in its full dimenfion in Plate VI. It has, like the 
others, a Aide made of fteel, of fuch thicknefs as to permit, the 
7 % micrometer 
