APPENDIX, No. III. 



77 



from the deductions which resulted from experi- 

 ments made by Captain Kater on a similar pen- 

 dulum. 



An astronomical circle, by Troughton, was used 

 as a transit instrument, and was so placed in a 

 small octagonal observatory of light pannels, com- 

 municating by a door with the tent, that the clock 

 could be seen, and its beats heard, by the observer 

 at the instrument ; thus, with the exception of the 

 first day's transits, the time was recorded directly 

 from the clock, without the intervention of a chro- 

 nometer. The meridian mark was placed near 

 the sea, at the distance of 806 feet : a strong post 

 having been driven into a cleft of the rock, and 

 firmly secured, a screen was nailed to it made 

 of copper, and perforated with a set of holes, from 

 one-fourth to one-tenth of an inch in diameter, 

 and readily distinguishable from the Observatory. 

 This fixed screen being made in the form of a box 

 to receive the lamp, it became impossible to mis- 

 place the light. ^The instrument was brought down 

 to this mark, and the level carefully examined, be- 

 fore and after every observation, except with some 

 stars which followed too close upon one another. 

 The sun was fortunately observed at noon every 

 day ; and as its rays were never allowed to touch 

 any part of the instrument, or to enter the Obser- 

 vatory, except at the moment of noon, and then 

 only through a small aperture, I had reason to hope 

 that none of the adjustments were, at this obser- 

 vation, ever deranged. As the great alternations 

 in temperature alluded to above might naturally 

 be expected to cause fluctuations in the going of 

 the clock, it was satisfactory to have a series of 

 frequently recurring tests, brought to bear upon 

 this essential particular. As the same precautions 

 were observed at every station, this account of 

 them will apply to the whole series of experiments. 



But in order that no higher than a correct es- 

 timate be formed of this insulated experiment, it 

 is right to describe the peculiar circumstances 

 under which it was performed. It was above all 

 to be regretted that we were so much limited 

 in time, that we could not engage in a fresh series, 

 either at the same island, or on some other lying 

 nearer the equator : but the service upon which 

 the Conway was employed, rendered it necessary 

 that our stay should not be longer at the Galapa- 

 gos than the 16th of January. Now, as we an- 

 chored at Abingdon's Island on the 7th at noon, 

 there were barely nine complete days in which 

 everything was to be done. We had to search 

 for a landing-place, which occupied a considerable 

 time ; to decide upon a station ; to rig up our 

 tents ; to build the Observatory ; then to land the 

 instruments, and set them up ; and as we had no 

 time for trials and alterations, everything required 

 to be permanently fixed at once. We were for- 

 tunate in weather during the first two days, when 

 our things were all lying about, and our habita- 

 tions ill assorted ; but on the third night it rained 

 hard, and the water which trickled through the 

 canvass caused us some discomfort, although we 

 fortunately succeeded in sheltering the instruments. 

 The heat during the day was not only oppressive 

 at the time, but very exhausting in its effects ; 

 and at night, although the thermometer never fell 

 below 73°, the feeling of cold arising from the 

 transition from 93°, to which it sometimes rose in 

 the day, was no less disagreeable. 



It was with reluctance that I left the neighbour- 

 hood of the equator, without having made more 

 numerous and more varied, and consequently less 

 exceptionable observations on the length of the 

 pendulum. It would, above all, have been desir- 

 able to have swung it at stations more nearly re- 

 sembling those with which its vibrations were to 

 be compared. Thus, the results obtained from 

 the experiments at the Galapagos, though curious 

 in themselves, are not so valuable for comparing 

 with those deduced in this country. The time 

 may come, however, when they may be rendered 

 more useful ; that is to say, should experiments 

 be made with the same pendulum at stations re- 

 mote from the Galapagos, but resembling them in 

 insular situation, in size, and in geological cha- 

 racter ; such as the Azores, the Canaries, St. 

 Helena, the Isle of France, and various other 

 volcanic stations amongst the eastern islands of 

 the Indian and the Pacific oceans. The advan- 

 tage of having it swung at the Cape of Good Hope, 

 and especially at the Falkland Islands, which lie 

 in the correspondent latitude to that of London, 

 and at various other stations on the main land, or 

 on large islands, is still more obvious. 



OBSERVATIONS MADE AT SAN BLAS DE 

 CALIFORNIA, 



San Bias is a sea-port town on N.W. coast of 

 Mexico, in latitude 21^°N. and longitude 105^° 

 W. and not far from the south point of California. 

 The experiments were performed under favour- 

 able circumstances, the sky being clear, the tem- 

 perature steady, and the rate of the clock uniform. 

 The station, indeed, was more elevated than could 

 have been wished, being 115 feet above the level 

 of the sea, on the summit of a cylindrical rock of 

 compact whin-stone, and measuring not more than 

 500 feet across, and nearly perpendicular in three 

 quarters of its circumference. 



The length of the seconds pendulum at San 

 Bias, by these experiments, appears to be 39.03776 

 inches, and the mean ellipticity gT3*5T* 



By a second series of experiments at San Bias, 

 the details of which are given by my coadjutor, 

 Mr. Henry Foster, the length of the seconds pen- 

 dulum is made 39.03881, and the mean ellipticity 

 Toi* 56"* ^he circumstances in this case, however, 

 were not so favourable as those of the first series, 

 being to one another in the ratio of 47 to 397, 

 or nearly as 1 to 8. This arose from the change 

 which took place in the weather at that period, 

 the sky being overcast, the temperature fluctuat- 

 ing, and the rate of the clock unsteady. 



RIO DE JANEIRO. 



Two extensive series of experiments were made 

 at this place, first by myself, and then by Mr. 

 Foster ; the total number of the factors in the first 

 case being 210, and in the second 452. The re- 

 sults agree with surprising exactness for operations 

 entirely unconnected. The length of the seconds 

 pendulum by my experiments, being 39.04381 

 By Mr. Foster, . . . .39.04368 



The mean ellipticity by my experiments is, ^ct-tt 

 By Mr. Foster, .... to*-tt 



The circumstances in both cases were favour- 



