Oct. 



12, 



1876] 



NA TURE 



531 



But when you wind the clock, you relieve the great 

 wheel from the strain of the weight, and the clock would 

 Slop if you did not introduce mechanism to prevent it. 

 Fig. 6 represents such mechanism. 



In this case the click L is fastened not upon the great 

 wheel GG but upon an additional ratchet-wheel, R2R2' 

 which rides loosely upon the axis of the great wheel. Its 



Fig. 6. 



teeth, which point in the reverse direction to those of the 

 first ratchet-wheel, pass under the long click K K mounted 

 within the clock frame, and so far as the drivmg power of 

 the clock weight is concerned, its action may be neglected 

 altogether. 



This ratchet-wheel is connected with the great wheel 

 only by the spring S S, one end of the spring being 

 fastened to the great wheel and ihe other to ihe ratchet- 

 wheel. The strain of the clock weight keeps this spring 

 closed and is transmitted to the great wheel through it. 



Let us see what will happen when we try to wind. The 

 spring S S is relieved from the strain of the weight and 

 essays to open by thrusting back the ratchet R2 R2, but 

 this it cannot do, for the long click K K prevents it, and 

 banking against this the thrust of the spring is transferred 

 to the clock-train. 



Other mechanism is also employed for the purpose. 



One very favourite plan (a very old one, which has been 

 once or twice re-invented lately) places the fulcrum of the 

 lever (in other words, the spindle of the wheel) through 

 which the barrel is wound, upon the great wheel itself. 



Great care has to be taken both in shaping and sizeing 

 the various wheels and pinions. It is an advantage 



to have high numbered pinions, because in this case you 

 do not get so oblique an action of the wheel teeth upon 

 the teeth of the pinions : the action is more across the 

 line of centres. 



The curves of the teeth must also be properly formed. 

 The broad principle is to get an uniform running, that is, 

 that the pinion shall always move at a fixed and definite 

 rate with regard to wheel, for if it moves faster or slower 

 it is quite clear that the wheel tooth is acting too far up 

 or too low down the flank of the pinion tooth, that is to 

 say, working it at the end of too short or too long a 

 lever ; and less or more power is accordingly transmitted. 

 If you look at Fig, 7 you will see easily that if the top of 

 the wheel tooth a were not rounded off quite so much it 

 (supposing the present curve correct) must drive the 

 pinion too fast, and too little power would then be 

 delivered. 



Sometimes the main clock-train is merely employed to 

 wind up at cenain short intervals (usually twice a minute) 

 a subsidiary weight or mainspring, which latter is that 

 which immediately propels the escape wheel. In this 

 manner variations in the friction of the clock-train can 



Fig. 8. 



be in great measure prevented from reaching the pendu- 

 lum, if there is a little less or moie power upon the 

 clock-train, the only effect being to wind up the subsidiary 

 weight or spring more or less rapidly. The main clock - 

 train is at the right moment liberated by some mechanism 

 upon the spindle of the escape-wheel and the minute- 

 hand being connected with it moves by jumps whenever 

 the weight or spring is wound up. 



The general arrangement of the train of watches and 

 chronometers differs little from that of clocks, but the 

 power is delivered by means of a coiled spring, which 

 necessitates the following arrangement. 



The spring pulls harder the further you wind it, and its 

 force at commencing would be obviously greater than 

 when it has in part run down ; we therefore introduce the 

 following compensation (see Fig. 8). We place the great 

 wheel upon that hollow-sided cone or " fusee " A A, and 

 connect it with the barrel B B (which is impelled by the 

 main spring inside it) by means of a chain, c. When the 

 spring pulls hardest it has the thinner part of the fusee to 

 act upon, it works a lever of shorter radius, and the force 

 at the circumference of the great wheel is in this manner 

 equalised. 



{To be continued.) 



FLORIDA SHELL MOUNDS^ 



THE river St. John drains the eastern portion of the 

 northern half of the peninsula of Florida, running 

 northward over a flat country for a distance of about 300 

 miles. In the lower part of Us course it opens out into 

 large sheets of water two to three miles in width, and as 

 might be expected from the nature of the country, it 

 frequently shifts its bed, and is liable to annual inun- 



' Fresh-water Shell Mounds of the St. John's River. Florida. By Prof. 

 Teffries Wyman. In the Memoirs of the Peabody Academy of Science. 

 Vol. I. No. 4. 



