$08 SCIENTIFIC NEWS. 



SCIENTIFIC NEWS. 



French National Institute, 



tute. JLVJlR. Delambre, perpetual secretary, has given an ana- 



lysis of the labours of the mathematical division of the class 

 of mathematical and physical sciences for the year 1807, of 

 which the following is a brief account. 



New construe- Mr. Burckhardt has proposed a mode of constructing 

 tio-iortele- ... .. , , - r . ... , , . & 



scopes, telescopes, whieh he conceives will render their use more 



easy and convenient, than any yet adopted. IJis smaller 

 mirror is plane, like Newton's, but placed perpendicular to 

 the axis of the large concave mirror, and at half its focal 

 distance. In this place the section of the reflected cone of 

 light is a circle, the diameter of which is just half that of 

 the large mirror. Accordingly the small mirror intercept* 

 a fourth of the direct rays, but Mr. Burckhardt compen- 

 sates this loss, by increasing the dimensions of the large 

 mirror a little. The cone thus intercepted takes an in- 

 verted direction; and the rays, instead of proceeding to 

 their focus behind the small mirror, unite at an equal' dis- 

 tance in front of it, passing through an aperture in tne cen- 

 tre of the large mirror. The telescope, thus reduced to 

 half its length, will have four times as much light as a com- 

 mon reflecting telescope of the same length. Many ob- 

 jections were made to this construction, which Mr. B. an- 

 swered, and it was agreed, that one should be made for 

 trial. 

 ^ lorda's circle. The astronomers, who have lately measured the meridian 

 f line between Dunkirk and Barcelona, have .employed Bor- 

 da's circle to determine the time for correcting their clocks. 

 They presume, that in an interval of four or six minutes, 

 during which four or six observations maybe taken, the al- 

 titude of the sun or a star increases with sufficient uni- 

 formity in proportion to the interval, so that a mean be- 

 tween the observations may be taken, and employed safely 

 as a sing-le observation. 

 Formulae for Mr. Delambre and Mr. Burckhardt give several useful 

 altitudes. formulae for taking altitudes of the stars, and likewise the 



moon 9 



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