Recognized HTML document

Memoranda and Log-Books.

31

impossible to correlate. A series should consist of six sets, each set including three simple distances. Three of these sets should be to a star or stars E. of moon, and three to a star or stars WWT, of moon. Lunars not taken on the E. and W. plan are almost worthless, no matter how numerous they may be, for the sextant, &c., might be inaccurate to any amount, and yet no error be manifest in their results. But the E. and W. plan exposes errors mercilessly, and also eliminates them. One of the best authorities on the requirements of sextant observations in rude land travel, the Astronomer Royal of Cape Town, says to this effect:-" Do not observe the altitude of the star in taking lunars, but compute it. The labour requisite for that observation is better bestowed in taking a large number of distances." So much delicacy of hand and of eyesight is requisite in taking lunars that shall give results reliable to seven or eight miles, and so small an exertion or flurry spoils that delicacy, that economy of labour and fidget is a matter to be carefully studied.

These things being premised, it will be readily understood that outline forms sufficient for an entire series of lunars will extend over many pages -they will, in fact, require eighteen pages. There are four sets of observations for time:-one E. and one W., both at beginning and close of the whole ; one for latitudes N. and S.; six for six sets of lunars, as described above ; six for the corresponding altitudes of the stars, which have to be computed ; and, finally, one page for taking means, and recording the observations for adjustment, &c. Each double observation for latitude would take one page ; each single time observation one page ; and each single compass variation one page. An occultation would require three pages in all ; one of which would be for time. At this rate, and taking the observations mentioned above, a book of 500 pages would last half a year. Of course where the means of transport is limited, travellers must content themselves with less. Thus Captain Speke, who started on his great journey amply equipped with log-books and calculation-boobs, such as I have described, found them too great an incumbrance, and was compelled to abandon them. The result was, the though he brought back a very large number of laborious observations, there was a want of method in them, which made a consider-