Captain Hall's Notice of a Voyage of Research. 359 



verted to. The ingenious contrivances of Captain Kater, have 

 already been shewn to be available in the hands of seamen, and 

 we know also, that the time requisite for the performance of 

 these interesting experiments is not great. 



There have already been a considerable number of such ob- 

 servations made by different observers, and with dfferent in- 

 struments. But the nature of the experiment is such, that this 

 circumstance, which in most other similar matters add to the 

 value of the work as a whole, in this case are not quite so 

 satisfactory ; for the experiments with the invariable pendulum 

 are so strictly comparative in their nature, that, in order to de- 

 duce any valuable conclusions from them, they ought to be used 

 by the same observer under similar circumstances, but in very 

 different situations ; so that the object in view, the determination 

 of the unequal figure of the earth, might be the sole cause of dif- 

 ference in the result. It would be highly desirable, therefore, to 

 ascertain the length of the pendulum at stations both near to 

 and remote from the equator, in the southern hemisphere, where 

 the question is fully of as much importance as in this. 



To investigate by actual trial the effect of local density, or that 

 which is caused by the nature of the ground at the station, on the 

 vibrations of a pendulum, has been considered a most interest- 

 ing desideratum. To accomplish this, however, it is essential 

 that the same instruments be used, swung at a series of stations, 

 lying not in different latitudes, as in the first case alluded to, 

 but along the same parallel, where, according to theory, the 

 number of vibrations of the same pendulum, after allowance for 

 temperature has been made, ought to be alike ; and consequent- 

 ly the amount by which they should be ascertained to differ, 

 would express the effect of this disturbing cause. Once ascer- 

 tained, this would become a valuable element in the reductions, 

 and would be applicable generally to every previous or subse- 

 quent experiment on the length of die pendulum. 



The measurement of the height of mountains, by means of 

 the barometer, in conjunction with levelling and trigonometri- 

 cal operations, and in different climates, such, for example, as 

 Teneriffe and Terra del Fuego, might, if done with care, fur- 

 nish useful data in a very interesting branch of geographical 

 inquiry. In a similar spirit, the sea might be fathomed, and 

 water brought up from great depths, — the height and velocity 



