[ oo J 
XLVI. 
METHOD OF OBSERVING THE ALTITUDE OF A CELES- 
TIAL OBJECT AT SHA AT NIGHT-TIME OR WHEN 
THE HORIZON IS OBSCURED. By J. JOLY, S8c.D., F.G.S., 
F.R.S., etc., Hon. Sec. R.D.S. 
[Read Frsruary 19; Received for Publication Fepruary 21; Published May 14, 
1902]. 
ir is a common experience to find clear skies at night-time, 
presenting to the mariner what would be valuable opportunities 
for observation, if at the same time the horizon was available. 
This has led to the invention of many contrivances, depending on 
gravity or gyrostatic stability, designed to afford an artificial 
horizon. Any contrivances controlled by gravity must, however, 
possess vibrational properties which will render their use unreliable. 
If controlled by kinetic stability or by magnetic force, the correct 
setting of the instrument to horizontality presents difficulties and 
uncertainties which have not yet been surmounted. 
The want being thus not easily supplied by mechanical con- 
trivances, I describe in this paper a very simple mode of making 
observations when the horizon is obscured—a mode which demands, 
it may be said, no special apparatus and no experience or know- 
ledge from the mariner that he does not already possess; while 
fairly approximate results are obtainable, which under such con- 
ditions as would necessitate the use of the method would doubtless 
possess considerable value. 
I assume that the vessel is provided with the usual rescue- 
signals, as certified by the Board of Trade. These signals, when 
thrown overboard (being first perforated), burn in the water with 
a bright white light, visible in clear weather up to five miles, and 
burning in all states of wind and water for about half-an-hour. 
To one of these is attached some three or four fathoms of 
marlin, with a small piece of scrap-iron or any other suitable 
object attached at its extremity. The signal, so fitted, will not 
drift appreciably with the wind when thrown overboard. Save for 
