280 Report of the Aslronomkal Society. 



would serve as a mode of comparison for those observers who, 

 with less poweri'ul instruments, might be more favourably situated 

 for making observations at this important pohit of the orbit of 

 the planets. 



The Council further represented to Captain Mali that it was 

 scarcely to be expected that a traveller, who is frequently changing 

 his situation, can make ma.\ fundamental observations in astro- 

 nomy; such as mav serve as a basis for future researches. But, 

 that he might do much in comparative astronomy; by taking 

 those elements as correct which have been determined by ol)ser- 

 vations made in fixed observatories, and comparing the objects 

 of research therewith: some examples of which have been already 

 alluded to ; and others were suggested for his consideration. 



But there was still another, and a very important point, to 

 which the attention of Captain Hall was requested (and the same 

 cannot be too strongly pressed on any future voyager, or settler 

 in distant climates, favourably situated for such inquiries) : 

 namely, to make regular observations on the tides, in favourable 

 situations for determining their theory. It is well known that 

 the tides, adjoining large continents and their contiguous islands, 

 are so affected with the various sources of error arising from the 

 situation of ihe harbour and the nature of the bottom of the ocean 

 for a considerable distance around it, that not only a very long 

 series of observations is required to destroy or compensate those 

 errors, and allow the true coefficients of the formuhe, for deter« 

 mining their value, to ap|)ear with any tolerable exactness ; but 

 also the coefficients themselves, so determined, are essentially 

 affected by such local peculiarities; and consequently incapable 

 of affording any thing bevond relative results. In order to ob- 

 tain results unaffected with these inappreciable causes of error, 

 the places of observation should, if possible, be chosen on small 

 islands shooting up abru])tly from an unfathomable depth in the 

 midst of a wide ocean, extending 30 or 40 degrees, at least, in all 

 directions ; or, at all events, a verv great distance from any large 

 continent. The islands in the Pacific and South Atlantic oceans, 

 which are bedded on coral banks or the effect of volcanic erup- 

 tions, are precisely of this nature. If we may trust the accounts 

 of voyagers, manv of these are mere vertical shafts, or insulated 

 columns, shooting at once from the very bottom of the ocean, 

 without shoals, or any gradual declivity. Round these, the tides 

 must rise and fall with perfect uniformity : and It is exceedingly 

 ))robable that, in these cases, a much shorter series of observa- 

 tions would be requisite for framing accurate results : and that 

 even those of a single month, in moderately calm weather, might 

 have considerable value in the present improved state of the 

 theory. The situation of the Qallapagos iblandj;, on which Cap- 

 tain 



