journal of the Polytechnic School at Paris. gy 



Question IV. By J. B. 

 GIVEN the time in which mercury is raifed to the boiling point by the heat of a furnace, 

 and the rate of cooling per minute, after it has been removed, to determine the heat of the 

 furnace, or that to which the mercury has been expofed. 



SCIENTIFIC NEIFS. 



JL HE particular memoirs contained in the four firfl cahiers of the Journal of the Poly- 

 technic School at Paris are thirteen in number*: two on ftereotomy; one on fortification; 

 fix on chemiftry ; three on general phyfics ; and one on the application to the arts. 



The memoirs of ftereotomy treat : l. On the determination of tints in defigns, by feveral 

 pupils of the fchool ; and 2. On the curve lines of the furface of the ellipfoid, by Monge. 



The folution of the interefting problem of the determination of tints is deduced from the 

 leffons which Monge has given upon perfpeftive. This folution depends on the funda- 

 mental principle, that all bodies refleft the white rays; and that the quantity of rays of this 

 colour, reflected from each point of a body, depends on its polifli, and the angles formed 

 by the incident and reflefted rays which come to the eye ; that the more white rays are 

 reflevSed, the more luminous the body will appear ; and on the contrary, the lefs of thefe 

 rays are reflcfled, the more obfcure it will be. 



All polifhed bodies prefent a white fpot or line ; the line is vifible upon cylinders, 

 cones, &c. ; the point, upon furfaces of double curvature. 



When bodies are perfeftly poliflied, a white point or determinate furface is perceived ; 

 but when they are obfcure, a fucceflive degradation of tints is feen, which depends on the 

 form of the body. 



From the point, the line, or the moft enlightened furface, it is pofllble to trace a fucceffion 

 of curves of equal tint : thefe are the curves of which the authors of this memoir have in 

 the firft place determined the equation, and afterwards fought to afcertain the law of the de- 

 gradation of the tints. 



If it be afTumed that any tint laid on white paper will poflefs an intenfity dependent on 

 the quantity of colour fpread, and the proportion of white points left uncovered, the 

 authors of the memoir have fought to determine whether the law of the application of tints 

 upon each other ought to depend on their degradation : by this means they have fucceeded 

 in giving the theory of waflied tints, properly fo called. 



The folution of this problem, fo interefting to thofe who cultivate the art of defign, and 

 are capable of purfuing the calculation it demands, proves the extended knowledge to which 

 the pupils have arrived, and the progrefs they daily make, under their Inftitutor. 



The memoir on the curve lines of the furface of the ellipfoid, which Monge has given in 

 the fetond cahier of the Journal Polytcchnique, contains an application of the property of 

 curve furfaces with regard to their curvature, to the configuration of ftones for arches. 



TJic joints of arch-work ought to be made agreeable to a number of Conditions j 

 the chief of which are: " i. They muft be everywhere perpendicular to the arch, 



• Tliit Analyfit u iranflattJ from the Journal dc» Sqavant, p. 117. 



