1 87 1.] Pneumatic Transmission. 309 



experiments to determine its value have been made by Girard, 

 D'Aubuisson, Buff, Pecqueur, and others, who give a mean 

 value for it of 0*02. 



Dr. P. Brix, Professor at the Bau-Akademie, has pub- 

 lished in the German "Telegraph T ournal" particulars of 

 experiments made by him upon velocity with a tube 2 J inches 

 in diameter, laid down some years ago, by Messrs. Siemens 

 at Berlin, between the Exchange and Central Telegraph 

 Station, the results of which were that when working with 

 compression the tension of the air at either end of the tube 

 was 19*31 lbs. and 1475 lbs., and with exhaustion, 14*75 lbs. 

 and 10*19 lbs., respectively; the mechanical effect produced 

 by one cubic foot of air in each case was 512*17 lbs. in the 

 former, and 520*44 lbs. in the latter ; and the weight of the air 

 at the two extreme tensions was, in compression, 0*1099 and 

 0*0753 foot-pounds, and in exhaustion 0*0752 and 0*0447 foot- 

 pounds. For each case the frictional resistance of the 

 carriers in the tube averaged o*i lib. ; the length, 2920 feet 

 for each half of the tube; its diameter, 0*193 feet; and its 

 volume, 85*49 cubic feet. With these values, the formula 

 worked out by Sabine gives the calculated speeds in these 

 two experiments as follows : — With compression, 34*1 feet, 

 and with exhaustion 43*2 feet per second, or that the carrier 

 should have occupied in the transit from station to station, in 

 the former case 86 seconds, and in the latter 68 seconds, dif- 

 fering from the observations made by Dr. Brix 9 seconds in 

 the one case, and only 2 seconds in the other. This dif- 

 ference, Sabine thinks, may possibly be due to an error of 

 observation of the pressure, or possibly to the fact that the 

 constant f may not be the same for small welded iron tubes as 

 for a large cast-iron tunnel. 



Mr. Sabine has also made some experiments with the 

 tube of the Pneumatic Company, between Euston Station 

 and High Holborn, which was some years ago designed by, 

 and carried out under the engineering superintendence of, Mr. 

 Rammell and Mc. Latimer Clark. This tube is d -shaped, 

 4J feet broad and 4 feet high. The trains used were each 

 made up of three trucks, and these were loaded with an 

 average weight of 6 tons, making, with the carriages, a 

 gross load of 9 tons. The average time occupied in running 

 through the tube from Euston Station to Holborn was 

 7J minutes, with a partial vacuum of 5 ozs. per square inch, 

 whilst the empty trucks were returned to Euston Station 

 with a compression of 5 ozs. per square inch in 65- minutes. 

 Assuming the temperature of the air to have been 20 C, and 

 its mean pressure 14*75 lbs., it is calculated that in drawing 



