“Si je donne ces détails, nouveaux, ou peu connus, c’est qu’il est toujours intéressant de remonter à l’origine et au développement successif des inventions.” (M. Berthelot, in the “Comptes Rendus.”) “S’il n’y a pas beaucoup de gens qu’elles intéressent, il y en a qu’elles intéressent beaucoup. À ceux-ci, nous avons, en rédigeant ces notes, eu l’intention et l’espérance de venir quelquefois en aide. Tout catalogue a des points obscurs, même les meilleurs.... L’office propre, le devoir de la critique, est de rechercher si ces points obscurs ne pourraient pas être éclairés par quelque lumière. Il est vrai qu’elle y perd souvent sa peine. Mais cela ne doît jamais la décourager.” (M. Hauréau, in “Le Journal des Savants.”) I am especially thankful for the warm encouragement which I have received, on all sides, since the original work appeared in serial form. This History has been frequently called for, and I regret that I have been hitherto prevented from bringing it out earlier in its present desirable book form. This is the more to be regretted as it long ago received the practical endorsement of the _doyen_ of the electrical profession, Lord Kelvin (formerly styled Sir William Thomson), to whom it is dedicated. Leave to do this was obtained through a mutual friend in such a cordial manner that I cannot refrain from giving the correspondence attaching thereto: _Westminster Chambers, London, S.W., January 4, 1894._ “DEAR MOTTELAY, “I duly received yours of the 21st ... but the point on which I feel guilty is your _dedication_.... I have now started the matter by writing to Lord Kelvin fully on the subject, and I hope, within a week, to get his reply, which I shall at once send to you--he cannot possibly wish to decline the honour.... “I remain, “Yours very truly, “LATIMER CLARK.” _Westminster Chambers, London, S.W., January 13, 1894._ “DEAR MOTTELAY, “Lord Kelvin’s letter is so nice a one that I send you the original, otherwise I should have liked it as an autograph for my library. I shall be glad to hear that it has duly reached you.... “Yours very truly, “LATIMER CLARK.” _The University, Glasgow, January 11, 1894._ “DEAR CLARK, “Your letter of the 4th should have been answered sooner, but for my absence from home at the time it came.