VOL. LXXXV.] PHILOSOPHICAL TRANSACTIONS. 62g 



but it sometimes happened, that only 1 of the 2 white lights, which were burned 

 at tlie distant stations, was seen; in which case, if the observation appeared to 

 be made without any error, except that which an inequality in the division of the 

 instrument might be supposed to produce, it was considered as sufficient: other- 

 wise fresh lights were sent to the station and observed. 



In the use of the white lights, it is conceived that sufficient precautions were 

 taken, as the firing of them was always committed to particular soldiers of the 

 party, selected from the rest on account of their capacity and steadiness, who 

 had instructions to place the copper nozle immediately over the point marking 

 the station, by means of a plumb-line let fall from the bottom. In observing 

 them with the instrument, the angle was not taken till the light was going out. 

 But the men commonly guarded against the flame being blown greatly on one 

 side, by erecting something to windward of the light. 



In the use of the lamps also, care was taken to give them their proper direc- 

 tion; for when the ground about the station would not admit of the lamp being 

 placed immediately on it, slender stafFs were erected supported by braces, and 

 made upright, by being plumbed in directions at right angles to each other. 

 Precautions were also used to put those stafl^s precisely over the points, by cen- 

 treing the holes in the cross-boards. 



In a very early stage of the business it was found, that the effects of heat and 

 cold on the limb of the instrument were likely to produce the greatest errors; 

 for if the canvass partitions, forming the sides of the observatory, were open to 

 windward, streams of air passing unequally over its surface would cause such 

 sudden effects, that little dependance could be placed on any observations made 

 with the instrument in such a state. To avoid this; it was the constant practice 

 when the wind blew with any degree of violence, to prevent the admission of it 

 as much as possible, by keeping up the walls of the external tent, leaving only 

 a sufficient opening for the discovery of the lamp or light; and at other times 

 when the wind blew moderately, and a greater difference appeared in the readings 

 of the opposite microscopes, than an error in division might be supposed to pro- 

 duce, the walls of the external tent were entirely thrown down, and the instru- 

 ment kept in an equal temperature by the admission of air on all sides. 



In taking the angles, it was a general rule for some person to keep his eye at 

 one of the microscopes, and bisect the dot, as the observer moved the limb with 

 the finger-screw of tlie clamp. This precaution is very necessary when white 

 lights are used; for should there be a mistake in reading off an angle, when se- 

 veral are taken from the same lamp as the permanent object, it sometimes may 

 prove troublesome to rectify the error, without sending other white lights to the 

 stations. We found that to be the case at Ditchling Beacon, when only one 

 person happened to be at the instrument, and a reading was set down 10* wrong. 



