30 ANNUAL OF SCIENTIFIC DISCOVERY. 



villages are of one piece with the back-scene ; and while nature pre- 

 sents objects to us through a cone of visual rays drawn from the eye, 

 the stage represents them in an exactly inverted position that is, Ave 

 see them through the base, instead of through the vertex of the cone. 

 By M. Foucault's ingenious and artistical plan, all these inconvenien- 

 ce's are obviated. The sky being so often required, 'he has made the 

 upper part of it fixed, of a dome-like shape, as in nature ; the lower 

 or perpendicular part is of canvas stretched on frames, and arranged 

 cylindrically, so as to form a panorama, the end of which cannot be 

 perceived from any point of the house. His mountains or towns of the 

 background are independent of the sky, and stand forth in real relief; 

 so do his trees or shrubs, which are made to rise from or descend below 

 the floor. As for those objects which are nearer the foreground, they 

 are made of two pieces, the lower one to sink down, the upper one, a 

 fly, to be drawn upward when a change of scene is required. His 

 views of the sea or of interminable plains display a vast expanse never 

 yet seen on a stage ; rich architecture is also cut out, and shows beau- 

 tifully on the fixed sky of the background, which, however, is so con- 

 trived that all the phenomena of storms, sunset, approaching night, 

 travelling clouds, with varying illuminations, etc., are imitated with 

 surprising fidelity. 



SQUARING THE CIRCLE. 



M. Hobinet has presented to the French Academy aperies of equa- 

 tions laboriously worked out and represented as having a remarkable 

 approximation to accuracy. The proportion of various multiples and 

 fractions of the diameter to the circumference is given, and it is stated 

 that " the side of a square equivalent to a circle of a diameter equal to 

 unity = Iff 0.000,0006 = 0.8862269." 



A NEW SLOPE-LEVEL, BY M. RIBOT. 



This instrument is designed to solve practically either of the follow- 

 ing inverse problems, namely, to find the slope per metre of a given line ; 

 or, to set a line to a given slope per metre (or per yard). The instru- 

 ment is so simple as scarcely to need description. The horizontal dis- 

 tance between the feet or points of support is exactly one metre (or 

 yard). The right hand foot of the figure is capable of protrusion by 

 a screw, and is provided with a scale to measure the amount of this 

 protrusion. When the two feet are on a level, the index is at the 

 zero of the scale. If you want to determine a given slope, project 

 the foot until the instrument stands on the slope ; the protrusion mea- 

 sured on the scale gives the slope in terms of the distance apart of the 

 feet (metres or yards). If you want to establish a given slope, set the 

 foot to the indicated point of the scale, and adjust your plane to the 

 instrument. The lower bar of the instrument may be graduated so 

 that the plummet shall read* angles of slope. Dull. Soc. d' Encour. 

 pour V Indus. Nationale. 



A NEW MODE OF CONSTRUCTING CASKS. 



A new method of constructing casks has been recently patented by 

 John Connolly, of Boston. It consists in making the heads of the cask 

 of iron, or other metal, so arranged that the iron head serves at once 



