1824.] On Steam-Engines. 351 



emplary uniformity in perpetuating the error. Mr. D. however, 

 seems " perfectly assured that they are fully competent to de- 

 fend themselves, if they think it worth while." 



With regard to the depuration of mercury, Mr. D. mentions 

 how I may extend my knowledge. I am myself a workman, 

 although of name too obscure perhaps, ever to have reached 

 Mr. D.'s ear; half a century, however, of experience in the 

 habitudes of mercury, has enabled me duly to appreciate all 

 available sources in information. 



Surely Mr. D. does not imagine that any personal allusion 

 was intended by the words mentioned in his concluding para- 

 graph ; they were used merely in odium philosophorum, and still 

 must I be permitted to think the expression most appropriate. 

 Even at this hoar no one knows with certainty whether mercury 



expands — or — , and in choosing a number each must be 



guided by vague predilection. 



Hoping, however, that Mr. Daniell and I part good friends, I 

 promise, in the event of his coming to take the altitudes of the 

 Grampian range near which I reside, that he shall be shown 

 how to detect the most minute impurity existing in mercury, by 

 inspection of a single drop of that metal. X. 



Article VIII. 



On the Advantages, the Inconveniences, and the Comparative 

 Danger of High, Mean, and Loiv Pressure Steam Engines. 

 (Extracted from the Report to the French Institute, by MM. 

 Laplace, Prony, Ampere, Girard, and Dupin.) 



The subject resolves itself into the two following questions : 



1. What are the relative advantages of mean and high pres- 

 sure steam engines ? and 



2. What is the danger that attends them ? 



First Part. — Comparative Advantages of High Pressure 



Engines. 



Amongst the advantages of high pressure engines, that of 

 occupying the least possible space must be enumerated, and 

 will be the more important, as the space for their erection is 

 more confined, or the ground more valuable : where manufac- 

 tories, and private houses, are so crowded together that each 

 establishment can obtain but a very limited space, and great 

 power is at the same time necessary, this advantage is particu- 

 larly felt; and it is no less important in the interior of mines, 

 for the same reason. 



