"French National Insiitiite, 1 83 



■iJuccd complete conviction, we have reason to think thiit 

 the irregularities of our globe arc not so great as has hi- 

 therto been believed, and that the curve of the meridian, 

 abstractino; some local circumstances, deviates much less 

 frona the regular elliptical figure. 



It is acknowledged by the ablest naval officers that the 

 pc^rt of Brest in time of war cannot be supplied with pro- 

 visions and stores by sea, and the burthensome method of 

 employing carts can be recurred to only on the luost ur- 

 gent occasions. The counsellor of state Bruix has already 

 proved in a printed memoir the indispensable necessity of 

 an internal communication between Brest and the Loire. 

 Boats of the burthen of ten tons at most, and a canal, 

 would be sufficient for the continual wants of the navy. 

 C. Rochon, who has been long occupied with projects of 

 internal navigation proposed to the states of Brittany, has 

 given more extent to his ideas in a memoir which he read 

 to the class. He shows in what manner a highly useful 

 communication might be established between Nantes, 

 L'Orient, and Brest, by rendering navigable the rivers 

 Jlidre, Isac, Ourt, Blavet, and Chateaulin. 



We shall take this opportunity to say a few words re.- 

 specting some experiments lately mad£ by C. Raehon with 

 a telescope, of whicli he gave a description, with an ac>- 

 count of its uses, in a memoir printed in the year 9. 



It is well known that rock crystal has the property of 

 double refraction, and of produeing two images. This pro- 

 perty C. PiGchou has found the ingenious means of con»- 

 verting to advantage. A prism of tliis crystal placed in the 

 interior part of a telescope causes two images of the ob- 

 served object to appear, and these images approach or re- 

 cede from each other according as the prism is brought 

 nearer to, or removed frojm, the eye. When the images 

 are brought inio contact, a scale engraved on the outside of 

 the telescope indicates to the observer how many- times the 

 diameter of the observed object is contained in the distance. 

 I fence, when the distance is known the diameter can be 

 vietcrmined, and the diameter when known will givp a suf- 

 ficiently correct idea of the distance. If you observe a ship 

 at sea, which you wish to conje up with or avoid, bring in 

 contact the two ij^iages ; if you are approaching the vessel 

 the two images will encroach on each other: but, on the 

 otlier hand, if the two vessels 3rc receding from each (^(her, 

 the images will be soon separated. It is easy to distinguish 

 the rale of the observed vessel, and by these means you 

 way know very nearly the dimensions of her masts: bring 



M i into 



