186 Lord Oxmantown on the construction of 



Reauni< (about 54° Fahr.) At the station of M. Zenz it was 

 1° 5 Ream. (35^° Fahr.) while at the mineral springs it was 

 28° (8.1° Fahr.) at the time of the first observation, and 24° 

 (86°) at the time of the second observation. . The first of these 

 observations gives 680 feet of difference of level for each oc- 

 togesimal degree, and the second only 680 feet. 



One of the most interesting results was a magnetic one. They 

 found that the magnetic intensity decreased 0",01 upon 9.^'' 

 for every 1000 feet of elevation; a result which M. Kupffer 

 considers as incompatible with the hypothesis of a magnetic 

 nucleus which gives a much weaker decrease. — Ann. de Chim. 

 Tom. xlii. p. 105. 



Aet. XV. — Account of a series of Experiments on the co7istru&- 

 tion of large Reflecting Telescopes, By the Right Ho- 

 nourable Lord Oxmantown, M. P. Communicated by the 

 Author. 



Having, at different intervals during the last three years, 

 tried a variety of experiments on the construction of specula 

 for large reflecting telescopes, perhaps some of the results 

 which I have arrived at may not be uninteresting to the scien- 

 tific public. 



In making these experiments, I have had two objects in 

 view, Jirst, to ascertain whether it was practicable to remove 

 any of the defects known to exist in the large reflecting tele- 

 scopes hitherto constructed ; and, secondly^ to simplify the 

 process necessary for the manufacture of good reflecting tele- 

 scopes of ordinary dimensions, so that the art might be no 

 longer a mystery, known to but few individuals, and. not to be 

 acquired, but after many years of laborious apprenticeship. 



A general statement of the results of my experiments will 

 enable those who are at all conversant with the use of tele- 

 scopes to decide how far I may have succeeded in effecting 

 anything useful. 



I propose to avoid as much as possible entering into detail. 

 Within the limits necessarily prescribed for a single article in a 

 periodical work, it would be impossible to do so with any ad- 



