31: ANNUAL OF SCIENTIFIC DISCOVERY. 



easily proportioned so as to cool without strain ; besides, the tire is a hoop 

 and binder in this case, rather than a rupturing force. Sixth, The expense 

 and time required to put on a new tire are less than half what are re- 

 quired to put on a new wheel. The latter process involves taking out the 

 axle, putting it into a press, forcing off the wheel, forcing on a new one by 

 screw or hydraulic power, and replacing the wheel. Putting on a new tire 

 where the wheel is outside of the journal, as on locomotives and some ten- 

 ders and cars, does not necessitate taking out the axle at all; the engine or 

 car is simply jacked up, a few nuts are unscrewed, the old tire is slipped off, 

 and a new one slipped on. Seventh, If the tire breaks, the car has still a 

 wheel to run on, the broad periphery of the centre-plate or nave being left, 

 which is less likely to cause running off the track than a broken solid wheel. 

 Eighth, Not only is the wheel vastly better in every respect than a solid 

 wheel, but the expense of this generally most expensive department of re- 

 pairs is greatly reduced. The price of a common wheel being thirteen dol- 

 lars and fifty cents, the price of the double wheel is sixteen dollars, of which 

 the tire, turned out and furnished with four bolts, costs nine dollars. Two 

 common wheels cost twenty-seven dollars, while the first double wheel and 

 a new tire for the old centre of the double wheel cost twenty-five dollars, 

 saving two dollars on the two sets of wheels. Supposing the old nave to 

 outlast three tires, six dollars and fifty cents are saved; and supposing it to 

 outlast six tires, which is a reasonable supposition, twenty dollars are 

 saved; that is, compared with the cost of six common wheels, the new 

 wheel will have cost nothing, and will have saved six dollars and fifty cents 

 besides. When these figures are applied to the thousands of car-wheels re- 

 newed yearly on our great lines, not to speak of the grand aggregate on all 

 the railways in the country, the -saving promised b} T this improvement is to 

 be represented by millions of dollars. N. Y. Times. 



ON THE ACTION OF HEAT-DIFFUSERS. 



The following paper has been communicated to the British Association by 

 Mr. Arthur Taylor: 



Mr. Williams and others have found that an increased effect was produced 

 by the fuel burnt in steam-boilers when what have been called Heat-Diffus- 

 ers were placed in the tubes or flues. The apparatus in question consists 

 generally of metallic bands or ribands, twisted into spirals, or bent in the 

 direction of their length into zigzag forms, and placed in the tubes or flues, 

 the professed object of this addition being to break up or disturb the current 

 of heated gases passing through the tubes, and to cause every portion of the 

 gases to impinge on the heating surfaces, the cause given for the increased 

 effect produced being, that when a current of heated gases passes through 

 a tube under ordinary circumstances, only the exterior portions of the col- 

 umns come in contact with the sides of the tube, and that in thus disturbing 

 the current by obstacles to its direct coui-se a more perfect contact of the 

 gases with the surfaces is produced. The question which I wish to raise 

 is, whether this is the true explanation of the effect produced by diffusers, 

 deflecting bridges, etc. I think it can hardly be admitted that each mole- 

 cule of a gas passing through a tube follows a course parallel with the axis; 

 for those in contact with the sides of the tube will be so impeded by friction 

 as to have a much slower motion than those in the centre; just as in a river 

 the current near the banks is less rapid than that in the middle of the 



