Mechanical Science. 143 



engine owed its failure in the steam-boat, not to bad workmanship 

 on the part of the Glasgow engineers. 



I am, dear Sir, 



Yours very truly, 

 Glasgow i March Is/, 1825. Andrew Ure. 



14. On Paratonnerrcsy or Conductors of Lightning. — A very in- 

 teresting report on the subject of Paratonnerres, has been presented 

 to the Royal Academy of Sciences by M. Gay-Lussac, in the name 

 of a commission appointed specifically for the purpose, and an ac- 

 count of which has since been published in the Annates de Chimie, 

 and more recently a translation has appeared in the Annals of Phi- 

 losophy., for Decem])er, 1824. The paper is divided into two parts ; 

 one theoretical^ and the other practical, and the information con- 

 tained in it may be regarded as the most perfect we possess on the 

 subject. 



The theoretical part is introduced with some general obser- 

 vations on electric matter, and of conductors ; that its velocity is 

 at the rate of about 1950 feet per second ; that it penetrates bo- 

 dies, and traverses their substance, with unequal degrees of velo- 

 city ; that the resistance of a conductor increases with its length, 

 and may exceed that which would be offered by a worse but 

 shorter conductor ; and that conductors of small diameter conduct 

 worse tlian those of larger. The electric matter also tends always 

 to spread itself over conductors, and to assume a state of equi- 

 librium in them, and becomes divided among them in proportion 

 to their form, and principally to their extent of surface ; and that 

 hence a body that is charged with the fluid being in communi- 

 cation with the immense surface of the earth, will retain no sen- 

 sible portion of it. 



Gay-Lussac defines a paratonnerre to be a conductor which the 

 electric matter prefers to the surrounding bodies, in order to 

 reach the ground, and expand itself through it ; and commonly 

 consists of a bar of iron elevated on the buildings it is intended 

 to protect, and descends without any divisions or breaks in its length, 

 mto water or moist ground. When a paratonnerre has any breaks 

 in it, or is not in perfect communication with a moist soil, the 

 lightning having struck it, flies from it to some neighbouring body, 

 or divides itself between the two, in order to pass more rapidly 

 into the earth. 



The most advantageous form that can be given to the extremity 

 of a paratonnerre is that of a very sharp cone, and Vie higher it 

 is elevated in the air, other circumstances being equal, the more its 

 efficacy will be increased, as is proved by the experiments of M.M. 

 de Romas and Charles. 



