L. MECHANICS AND ENGINEERING. 449 



tei- power to resist. The greater the dish the stronger the 

 wheel will be to resist the lateral strain ; but no more dish 

 should be given than necessary for the safety of the spoke, 

 by reason of other considerations. iVmong these considera- 

 tions the principal consists in this, that by reason of the disli 

 form the sole of the wheel must be leveled, making the 

 wheel conical in form instead of cylindrical. The wheel, 

 therefore, tends to describe a circular path around the point 

 where the prolongation of its sole would meet the prolonga- 

 tion of its axis. In every vehicle the wheel is compelled to 

 move straight forward ; consequently, instead of rolling, it 

 partly slides, producing thereby a friction between the ground 

 and it, and tending thereby to defeat the primary object of 

 using wheels. This latter point has been minutely investi- 

 gated by Colonel Close, who states that the force w^asted in 

 overcoming this friction varies directly as the square of the 

 breadth of the tire, and as the tangent of the hollow of the 

 axle-tree arm of the wheel, but inversely as the height of the 

 Avheel. If, then, on grounds of strength, a certain hollow- 

 ing of the axle-tree arm is unavoidable, we see how essential 

 it is to confine the width of the tire within the lowest allow- 

 able limits. Colonel Close states that, as the results of sev- 

 eral years' experience in the manufacture of heavy carriages 

 for army purposes, he is led to the following considerations : 

 First, in the case of carriages loaded heavily and drawn over 

 irregrular and broken o-round, a dish is of the utmost value 

 to strengthen the wheel, whether the carriages be on springs 

 or not. Second, in carriages not supplied with springs, and 

 liable to carry dead loads over all sorts of country, the lower 

 spoke should be almost, but not quite vertical. Third, if the 

 carriage be on easy springs, considerable dishing of the wheels 

 is admissible, without hollowing the arm. The wheels would 

 then be cylindrical, and would roll truly. In this case the 

 strut of the lower spoke is equal to the dish of the wheel. 

 3Iinutes of Royal Artillery Institution ^Yl\l.^ 372, 418. 



SCIENTIFIC BALLOONIXG. 



Since the death of Croce-Spinelli and Sivel, the taste for 

 balloon ascensions, so for from diminishing, seems to have 

 received a new impetus in France, in all parts of which coun- 

 try ascensions have of late taken place very frequently. The 



