The Country of the Dwarfs

by Paul B. Du Chaillu

I would take astronomical observations whenever I could, so that I might know my latitude and longitude, and I took a great many at Mayolo. In the evening I would bring out my sextant, my policeman's lantern, my artificial horizon, my thermometer, and would work for hours.

I will explain to you the use of the artificial horizon. It is so called on account of being an imitation of the natural horizon. Quicksilver is the best material. The heavenly bodies are reflected upon it, and you must lay your artificial horizon in such a way that the object you are watching is reflected on it, and then, with your sextant, you bring the direct object to its reflected image on the quicksilver, and the reading of the sextant gives you the number of degrees, minutes, and seconds of altitude.

It is always good to take two stars, one north and the other south of the zenith of the place. While at Mayolo I would often take one of the stars of the constellation of the Great Bear and one of the constellation of the Cross the same evening. You have to watch carefully when the star has reached its highest altitude, that is to say, when it appears neither to ascend or descend.

But the most difficult observations were those of the lunar distances for longitude. In those observations I generally used three sextants, one for the altitude of the moon, another for the altitude of a star, and another for the distance between the moon and the star. My watch, my slate, my pencil, and my policeman's lantern were also placed near me. The two artificial horizons were in front of me, and when every thing was ready I would take an altitude of the moon, then that of the star, then look at my watch, and note down the exact time of each observation; then take four distances, and note the exact time each distance was taken, and then again the altitude of the star and moon in the reverse order of the first portion of the observation.

The following example will show you how a lunar distance is taken with a sextant:

OBSERVATIONS FOR LUNAR DISTANCES.
Date.	Place.	Time.			Object.	Alt. and Distance.			Index Error		Temp.	Resulting Longitude, E.
1865, May 6	Máyolo (contd.)	H.	M.	S.		°	'	"	'	"	Fahr.	°	'	"
		11	1	30	☾ Alt.	121	12	40	on 6	30		11	7	15
		11	4	30	Jupiter Alt.	62	44	20	on 5	20		"
		11	7	25	Distance	85	43	40	on 0	40		"
		11	9	42	Distance	85	42	50	"			"
		11	11	53	Distance	85	42	20	"			"
		11	13	27	Distance	85	42	20	"			"
		11	15	10	Jupiter Alt.	67	31	0	"			"
		11	18	2	☾ Alt.	113	5	10			77.0	Planet E. of moon
"	"	11	19	44	☾ Alt.	112	16	0	on 6	50		11	11	15
		11	22	7	Jupiter Alt.	70	37	40	on 5	20		"
		11	24	24	Distance	85	38	0	on 0	40		"
		11	26	18	Distance	85	37	50	"			"
		11	31	43	Distance	85	37	0	"			"
		11	33	10	Distance	85	36	0	"			"
		11	35	8	Jupiter Alt.	76	22	0	"			"
		11	36	40	☾ Alt.	103	59	30			77.0	Planet E. of moon

Take as many lunar observations as you can east and west of the moon—the more the better—and you will be able to know your exact longitude with more certainty. It would be here too complicated to tell you how to make the calculations, but I am sure that after a while many of you would be able to make them.

By lunar observations, if sickness or some other cause has made you forget the day of the month, or even the year, you can find it again. Several times I lost my days while traveling.

The heat was intense at Mayolo. The rays of the sun were very powerful, and raised the mercury nearly to 150°. Just think of it! In order to know the heat of the sun, the thermometer was only a glass tube supported by two little sticks. I had to take care that the rays of the sun fell always perpendicularly on the mercury.