Heale. — On Moon Occultations. 57 



considerable amount of raw material could be obtained at small cost, to the 

 joint benefit of the agi'iculturist and the manufacture!-. Wrapping paper has 

 long been manufactured by the Americans from the flowering sheatlis of maize, 

 but this material could scarcely be obtained here in sufl3.cient quantity to be 

 made available by the manufacturer. 



Aet. XL — On the Prediction of Occultations of Stag's hy the Moon. 



By T. Heale. 

 [Bead before the Auckland Institute, 10th November, 1873.] 

 All the methods in use for ascertaining the longitude, independently of 

 chronometers, depend upon the observation of the moon's position at a certain 

 instant of time at the place, then ascertaining from the tables of the moon in 

 the naiitical almanac, or other similar publications, the instant of time at 

 Greenwich, or other standard position, at which she reaches that point. The 

 difference between the two times so obtained is the difference of longitude 

 between the two places. 



The most complete, as well as the most simple, method of making this 

 comparison, and the one almost invariably used for observatory purposes, is 

 to note the exact time of the moon's crossing the meridian of the place by the 

 transit instrument, taking, at the same time, her zenith distance, or not, 

 according to the instrument employed. 



But to effect this in at all a satisfactory measure requires an observatory 

 and fixed instruments of an expensive charactei-, and accurate observations 

 kept up for a considerable period and elaborately reduced by computation. 

 It is, therefore, inapplicable to the purposes of a traveller, either by sea or 

 land. The method chiefly employed when an approximate result has to be 

 obtained from a single set of observations, is by observing the moon's angular 

 distance fronr the sun, a planet, or a fixed star, commonly called lunars. 



The chief objections to this method depend on the circumstance that 

 since the moon at fastest moves only about 1 second to 24 seconds of longitude, 

 and ordinarily much less, every second of error in the angular measurement 

 produces an error about thirty times as great in the longitude ; and as the 

 observations have often to be taken in very inconvenient postures, in which 

 only light instruments held in the hand are available. On board ship accuracy 

 cannot, as a rule, be expected from them, and in practice they are now bvit 

 little used — far less frequently, as far as my observation goes, than they used to 

 be forty years ago, though the trouble of computing them has been greatly 

 lightened by special tables. 



The only remaining method of importance is by occultations of fixed stars, 



H 



