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NATURE 



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chronographs referred towns a cylinder (or barrel) 12 in. 

 in diameter, and 2 ft. 6 in. long. That cylinder, which 

 was covered with paper, rotated once in two minutes 

 Beneath it (see Fig. 1) was a pricker placed in electri- 

 cal connection with the standard clock, and alongside 

 another pricker placed in electrical communication with 

 \n observer at any of the instruments. At every second 

 of the standard clock, the clock pricker rose and punctured 

 the paper. Meanwhile, as the cylinder rotated, the 

 carriage, K, on which the prickers were mounted slowly, 

 travelled along the length of the cylinder ; and this motion 

 of k, combined with the rotation of the cylinder, caused 

 the succession of clock pricks to arrange themselves 

 around the cylinder in the form of a spiral. The time of 

 any observation was reckoned by comparing the puncture 



K 



■M" 



JF 



of the observation pricker with the two adjacent clock 

 pricks. The distance between each successive turn of the 

 spiral of clock pricks was & inches, and it was within this 

 space (which was limited by.'the consideration of the size of 

 the cylinder, and the number of hours of observation it 

 should contain) that the two prickers worked. 



In the Brussels chronograph, by the directions of 

 M. Houzeau, the Belgian Astronomer-Royal, provision 

 had to be made for three observation prickers, in addition 

 to the clock-pricker. The space available for the prickers 

 to work in was only Jin., and it was obviously impossible 

 to place them side by side. The difficulty was sur- 

 mounted by arranging them in the form of a fan, so that 

 they should converge into the space, which then became 

 amply sufficient for the disposition of the punctures. 



N u No, n,. x 4 (see Fig. 2), are the prickers. As may be 

 seen, they take the form of pins with very large heads. 

 Each is mounted in a sheath, S S. Each sheath is 

 jointed (see side section), and swings about an axis A a. 

 It is kept to its bearing by a spring. This arrangement 

 allows the pricker to swing forward a little as it enters the 

 moving paper. It corresponds to the action of the old 

 form of pricker shown in Fig. 3. The pricker, however, 

 that we are describing has an important advantage. 



It might happen that an observer on pressing down the 

 electric button which worked the pricker, would keep his 

 finger on it. In that case, with the old form, the pricker 

 would be kept against the paper, and would very likely 

 cause damage. But in the new case nothing of the kind 

 would happen, for each pricker, N„ N 2 , N 3 , n 4 , is projected 

 by the blow of its corresponding striker, T,, T 2) T 3 , t 4 , and 

 travelling beyond the reach of the striker, pierces the 

 paper by its own momentum only. On falling back, 



should the striker, T, be still kept raised, the pricker will 

 rest upon it, but its point will be free of the cylinder, and 

 at some distance, r>, below it. The strikers, T„, &c, 

 are worked by electro-magnets : the spiral spring shown 

 just above the head of each pricker, is compressed when 

 the pricker is projected between the head and sheath, and 

 assists in the disengagement of the pricker from the 

 paper. The punctures of the prickers are very marked 

 and distinct. 



A NON-ELECTRIC INCANDESCENT LAMP 

 A BRIGHT light, easily obtained and sufficient for 

 •**• projections, has frequently been regarded as a 

 desideratum, where it has been impracticable to procure 

 either the electric or the lime-light. The French Minister 

 of Public Instruction lately appointed a special commis- 

 sion to indicate the apparatus most suitable for projection 

 in primary schools ; and it appeared that while there was 

 no lack of simple arrangements for the projection proper, 



