532 PHILOSOPHICAL TRANSACTIONS. [aNNO 1732. 



with both hands in such a manner as is attest for keeping it steady, 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, cause the sun's image to fall on the 

 hole in the shield, so that it may rest in the centre of the sun's image; the 

 instant the azimuth tube and sun's image are thus regulated, see if the bubble 

 in the horizontal tube on the index (v%hich till then is disregarded) leaves the 

 open end of the tube, or stops any where clear of the ends of the tube: if 

 these happen at the same juncture, the altitude is then truly taken; but if the 

 bubble had remained in the inclosed end of the tube, when the azimuth bubble 

 and sun's image were regulated, the index must have been slid up; and if tar- 

 ried in the open end, moved down, till the horizontal bubble on the index 

 quit the open end of the tube, or stop between the ends, as was before observed; 

 and then is the quadrant set. In continuing the observation for a meridian al- 

 titude, the quadrant being set, as the sun rises, the horizontal babble on the 

 index, will not quit the open end of the tube, or stop between the ends, but 

 hang there, or leave it after the azimuth bubble and sun's image have been re- 

 gulated; which will require the index to be continually moved down, in order 

 to keep the quadrant set. When the sun is up, or on the meridian, the qua- 

 drant will remain set for some time; and on the sun's falling, the horizontal 

 bubble will have a reverse tendency, inclining or running wholly to the inclosed 

 end of the tube. 



To take the Altitude of the Sun or Stars by a fonvard Observation. — In 

 this method, the lens and tube on the index are disregarded. Hold the qua- 

 drant vertical ; and looking through the eye-vane, direct the axis or upper edge 

 of the shield to the sun or star ; if the axis cut the sun or star at the same 

 instant that the bubble in the horizontal tube on the shield quit the open end, 

 the altitude is then truly taken, and the quadrant set. But if it should leave 

 the open end of the tube before the axis or upper edge of the shield cut 

 the sun or star, then the eye-vane (or which is the same, the index) must be 

 slid down ; and if it remain at the open end, or quit it when the axis is above 

 the sun or star, moved up, till the quadrant is set. In continuing the observa- 

 tion for a meridian altitude, as the sun or star rises, the bubble in the horizon- 

 tal tube will always quit the open end of the tube before the axis cut the object; 

 so that to keep the quadrant set, the eye-vane must on every such alteration be 

 constantly moved down ; while the sun or star is on the meridian, the quadrant 

 will remain set; and when the sun or star falls, the bubble will act contrary to 

 what it did in the rising, resting wholly in the open end of the tube. 



To take the Suns Altitude with the Horizon. — Turn the back of the arch 

 towards the sun, and cause the sun's image to fall on the hole in the shield^ 



