Mechanical Science. 421 



Art. XVIII. MISCELLANEOUS INTELLIGENCE. 

 I. Mechanical Science. 

 § Astronomy, the Arts, Sfc. 



1. Improved Signal fur Trigonometrical Measurements. — A 

 new kind of signal has been used by M. Gauss of Gottingen, in 

 a trigonometrical measurement undertaken in Hanover, which 

 appears to possess many advantages. It consists of reflected 

 solar light: that astronomer having remarked that the light 

 reflected by a small plane mirror, was sufficiently intense to be 

 observed at greater distances than those of the sides of his greatest 

 triangles, had a couple of instruments made which he called 

 Heliotropes, and which, though simpler in their construction 

 than the Heliostat of S'Gravesande, were, like it, intended to 

 reflect the sun's rays in a constant direction. Whilst these instru- 

 ments were preparing, M. Gauss made use of Hadley's sextant, 

 which, for this purpose, was mounted on a solid foot iu the fol- 

 lowing manner: The plane of the instrument being inclined to 

 the proper degree, the angle between the sun and the terrestrial 

 object to which the sun's rays are to be reflected by the moveable 

 or great mirror of the instrument is to be observed. Then, without 

 altering the apparatus otherwise, the arm which carries the 

 mirror is to be moved, until the index is at double the angle 

 observed, when the sun's rays will be reflected on the spot desired, 

 so that from that point the image of the sun may be seen in the 

 mirror like a star. 



The same result may be obtained by previously fixing a third 

 mirror above the moveable mirror, on the same arm, and perpen- 

 dicular like it to the plane of the instrument; but which makes, 

 ■with the plane of the great mirror, an angle equal to the comple- 

 ment of 90 degrees of the angle formed by the visual ray with 

 the plane of the third mirror. When, with a sextant thus pre- 

 pared and fixed on a foot, the distance of the object from the 

 centre of the sun is observed, this third mirror reflects the sun's 

 rays at the same moment on the object itself, and an observer 

 being there, the signal will be the light reflected from the sex- 

 tant. It is easy, by a little practice, to give that motion to the 

 mirror wiiich is necessary to throw the sun's rays for some time 

 on the same object ; for, in consequence of the imperfection of 

 the mirror, the field over which the rays are reflected is large 

 enough to compensate for any little irregularity in the move- 

 ment. 



M. Gauss has found that mirrors, two inches by an inch and a 

 quarter, are quite large enough for these purposes : In some 

 experiments, made with a view of estimating the distance to which 



