Nov. 28, 1859.] G ALTON'S SUN SIGNALS FOR TRAVELLERS. 17 



the instrument with the left hand across its middle in such a way 

 as to leave a finger and thumh free to move the mirror. Next, 

 holding the right hand, as a screen, against the end of the instru- 

 ment, the latter is raised nearly to the eye and directed as justly as 

 possible towards the object aimed at. Then, rotating the whole 

 instrument with the hand and moving the mirror backwards and for- 

 wards with the fingers, the flash is watched, until it is seen to fall 

 upon the palm of the right hand. Finally, the instrument is rotated 

 carefully, and the mirror gently moved, until the flash falls as full 

 and fairly on the hand as it is possible to direct it : when this is the 

 case, on raising the tube to the eye, the image of the sun is almost 

 sure to be caught on its field. The instrument is fully as easy to 

 work with as a sextant. Without stirring the mirror, there is power 

 of flashing to any desired point within a large area, by slightly 

 rotating the instrument and following the image ; also by making 

 contact in different parts of the field of view. Eapid, passing flashes 

 are barely visible, for the mirror must be held steadily during an 

 appreciable time in order to be seen to full advantage. 



The heliostat can be mounted on a rest, which may screw at 

 will, either into any piece of wood or into the top of a photograph 

 tripod stand, as shown in Fig. 4. For long continued signalling, 

 a stand is undoubtedly convenient, k gives ample rotatory 

 movement ; t allows a sufficient movement in altitude, and z 

 complete movement in azimuth. No counterpoises are needed for 

 an instrument of ordinary size. The looking glass used for the 

 mirror must have its sides truly parallel. It may have a narrow 

 rim of silvering removed all the way from round its edge, and be 

 cemented on to the top of a shallow glass tray. In this way the 

 silvering is hermetically sealed from danger; and if the common 

 diamond cement be employed, no heat or ill usage can separate them. 



The instrument may be used simply to attract attention at great 

 distances or to convey a few simple signals, as single flashes for 

 affirmation, doublets for negation, and so forth : for this purpose the 

 small instrument Fig. 2 would fully suffice, or letters and words 

 can be signalled by adopting the well-known notation of Morse's 

 electric telegraph. It is necessary, before beginning, to have some 

 vague general idea where the intended correspondent is situated, 

 then to sweep the distant landscape with the flash, and await his 

 answer. As soon as this has been made, communication can be 

 carried on as long as may be desired. A long line of horizon can 

 be swept, iJackwards and forwards, with perfect ease, and it is found 

 to be just as easy to attract the attention of a con-espondent, whose 

 position is unknown, to 5 or 10 degrees, as when it has been ascer- 



VOL. IV. c 



