( 17 5 ) 
Rule for either backward or forwardObfervationS. 
If the Altitude does not exceed 30°, the Label mull 
be placed at the Station on the Radius or longeft Limb 
of the Quadrant ; if the Altitude is between 30® and 
60°, at the middle Station ; and if the Altitude exceed 
6o° at the uppermoft Station. 
7 " 1 take the Sun's Altitude by a backward Ob- 
fervation. 
This is done without ufing the Sight-Vane or Hori- 
zontal Tube on the Shield. Hold the Quadrant with 
both Hands in fuch a manner as is apteft for keeping it 
Ready, the Back of the Arch being turned toward the 
Sun. When the Bubble of the Azimuth Tube is 
brought under the Hole in the Shield, caufetheSun’s 
Image to fall on the Hole in the Shield, fo that it may 
reft in the Center of the Sun’s Image ; the Inftantthe 
Azimuth Tube and Sun’s Image are thus regulated, 
fee if the Bubble in the Horizontal Tube on the In- 
dex (which hill then is difregarded) leaves the open 
End of the Tube, or flops any where clear of the Ends 
of the Tube: If thefe happen at the fame Jundure, 
the Altitude is then truly taken 3 but if the Bubble 
had remained in the enclofed End of the Tube, 
when the Azimuth Bubble and Sun’s Image were re- 
gulated, the Index muft have been Hid up ; and if tar- 
ried in the open End, moved down, until the Hori- 
zontal Bubble on the Index quit the open End of the 
Tube, or flop between the Ends, as was before ob- 
ferved ; and then is the Quadrant fet. In continuing 
theObfervation for a Meridian Altitude, the Quadrant 
