158 



PHOTOGENIC DRAWING. 



made with a smallpaf t of its leng;th cy- 

 lindrical, and the remainder laperinc^con- 

 icall}' with a spiral thread cut into it, after 

 the manner of the fuzee of a watch; the 

 sponge is suspended hy a fine silk thread 

 to the cylindrical part of this axis, upon 

 which it winds. This is balanced by a 

 small weight suspended also by a thread, 

 wJiich winds upon the spiral fuzee. 



Then when the sponge grows heavier 

 in moist weather it descends and turns 

 the axis, and so draws up the weight; 

 which coming to a thicker part of the axis, 

 it becomes a balance to the sponge, and its 

 motion is shown by an attached scale and 

 vice versa, when the air becomes drier. 



Salt of tartar, or any othea" salt, or pot- 

 ashes may be put into the scale of a ba- 

 lance, and used instead of the sponge. 



Hyperbola in geometry, the section of 

 a cone made by a plane, so that the axis 

 of the section, inclines to the 0])posite 

 leg of the cone, which in the parabola is 

 parallel to it, and in the ellipsis intersects 

 it. The axis of the Hyperbolical section 

 will meet also, with the opposite side of 

 the cone, when produced above the ver- 

 tex. 



Ht/perboUc cylindroid, is a solid figure, 

 whose generation is given by Sir Christo- 

 pher Wren, in the Philosophical Trans- 

 actions. Thus, two opposite hyperbolas 

 being joined by the transverse axis, and 

 through the centre, a right line Toeing 

 drawn at right angles, to that axis; and 

 about that, as an axis, the hyperbolasbeing 

 supposed to revolve; by such revolution, 

 a body will be generated, which is called 

 the hyperbolic cj^'lindroid, whose bases, 

 and all sections parallel to them, will be 

 circles. In a subsequent transaction, the 

 sameauthor applies it to grinding of hyper- 

 bolical glasses; aiiirming that they must 

 be formed in this way, or not at all. Hy- 

 perbolic leg of a curve, is that which ap- 

 proaches infinitely near to some asymp- 

 tote. Sir Isaac Newton reduces all curves, 

 both of the first and higher kinds, into 

 those with hyperbolic legs, and those 

 with parabolic ones. 



Hyperbolic line is used by some au- 

 thors for what we call the hyperbola it- 

 self. In this sense, the plane surface ter- 

 minated by the curve line, is called the 

 hyperbola, or hyperbolic space; and the 



curve line that terminates it, the hyper- 

 bolic line. 



Hypothenuse, in geometry, the longest 

 side of a right-angled triangle ; or it is 

 that side which subtends the right angle. 



Euclid, lib. 1, proportion xlvii. de- 

 monstrates, that, in every rectilinear right 

 angled triangle, the square of the hypo- 

 thenuse is equal to the squares of botfi the 

 other sides. 



This celebrated problem was discover- 

 ed by Pythagoras, who is said to have 

 sacrificed a hecatomb to the muses, in gra- 

 titude for the disQovery. 



PHOTOGENIC DP.AWING. 



At the last sitting of the Academy of 

 Sciences, M. Arago announced one of the 

 most important discoveries in the fine arts 

 that has distinguished the present century,- 

 the author of which has already acquired 

 universal reputation by his miraculous 

 diorama — M. Daguerre. It is well known 

 that certain chemical substances, as 

 chlorate of silver, have the property 

 of changing their color by the mere con- 

 tact of light; and it is by a combination 

 of this nature, that M. Daguerre has suc- 

 ceeded in fixing upon paper prepared with 

 it, the rays that are directed on the table 

 of the camera obscura, and rendering the 

 optical tableau permanent. The exact re- 

 presentation of whatever objects this in- 

 strument is directed to, is, as every body 

 is aware, thrown down with vivid colors 

 upon the white skreen, prepared to re- 

 ceive them, and the rays of light that are 

 thus reflected have the power of acting 

 in the way above alluded to, on chlorate 

 of silver or certain preparations of it. 



In this manner an exact representation 

 of light and shade, of whatever object 

 may be wished to be viewed, is obtained 

 with the precise accuracy of nature her- 

 self, and it is stated to have all the soft- 

 ness of a fine aqua-tint engraving. M. 

 Biat compares it to the retina of the eye, 

 the objects being represented on one and 

 the other surface with almost equal ac- 

 curac5\ 



" What is the secret of the invention ? 

 What is the substance endowed with 

 such astonishing sensibility to the rays 

 of light, that it not only penetrates itself 

 with them, but preserves their impres- 



