Perusal of a late Publication by Mr. Babbage. 49 



suit depends. My object was different; it was to employ 

 Lunars as a means of accurate determination of longitude on 

 shore; and with that view I neglected nothing that could con- 

 duce to accuracy, either in the observation or in the result. 

 I may add, that there can be few naval officers who have had 

 so much experience, in that kind of Lunar observation, as I had 

 already gained before I commenced the observations in 1822, 

 having before that time published the results of more than two 

 thousand such observations. Of the seven stations the longi- 

 tudes of which I determined by Lunars in 1822, one only, so 

 far as I know, has been since examined, and the result pub- 

 lished. I made the longitude of the Barrack Square in the 

 Island of Ascension 14° 23' 46"*5 W., being about ten miles 

 west of the longitude given by Professor Lax, as furnished to 

 him by the Hydrographic Office, and nearly twenty miles west 

 of the longitude assigned in the Connaissa?ice des Terns ; which 

 two publications might with propriety be regarded as the best 

 authorities of that time. Capt. Duperrey, who visited Ascen- 

 sion in 1825, with pendulums, made the longitude of the Bar- 

 rack Square, by chronometers from St. Helena, 14° 24' 05'''7 ; 

 and from Tarifa 14° 24' 21 "'2, both determinations being 

 within one mile of mine, and both still more to the westward. 



4. Those who are conversant with experiments with inva- 

 riable pendulums know that the accord of partial results is 

 dependent, not on the skill of the observer, — for the method 

 is such as to render the observation nearly independent of in- 

 dividual skill, — but on the quality of the clock in keeping an 

 uniform rate in short intervals, and on the means of preserv- 

 ing an equable temperature. It is difficult to institute a just 

 comparison in these respects between the different circum- 

 stances of different observers. 



There is one case, however, though it is a rare one, which 

 does permit a comparison; it is when the circumstances of two 

 observers have been exactly the same. The last published 

 part of the Phil. Trans., containing Capt. Ronald's observa- 

 tions, with No. 4 Pendulum in London, affords such an op- 

 portunity. Capt. Ronald was furnished with the same inva- 

 riable pendulum which I had previously employed ; he tried 

 it in the same room, and with the same clock that I had used ; 

 the circumstances were as nearly the same as possible, ex- 

 cept that it was the first time (at least I believe so, for I was 

 not then in London) that Capt. Ronald had observed with an 

 invariable pendulum. Of eleven results obtained by him in 

 London, there are eight of which the difference from the mean 

 does not exceed one-tenth of a vibration in twenty-four hours ; 

 and the three others are all within one-fifth of a vibration, per 



N. S. Vol. 8. No. 43. July 1830. H diem, 



