No. 199.1 355 



Of these powers the lever is the most prominent in utility, and was 

 probably the first and most ancient attempt to accomplish by a machine 

 what was before the work of the unaided hand. It is well defined to 

 be " a body capable of turning about a fixed axis and acted upon by 

 forces." The common application of this poAver as pump handles, 

 arms of machinery, &c., are familiar ; I shall only notice here the 

 adaptation of the bent lever to the purposes of weighing, as shown in 

 the platform scales in exhibition ; in it the long ann is the arm on which 

 the dish is hung, the short arm, bent, is concealed in a box and turns 

 an index needle which moves through a large arc where the weight 

 is read off. The advantage of the bent lever is this — that by its pres- 

 sure is communicated rapidly, and as rapidly removed, and in its appli- 

 cation to the balance, the least weight in the scale dish is immediately 

 read oflf on the index arc. 



Wheels are only modified levers, and exert the power of straight 

 levers more advantageously, and in a continuous manner : thus the 

 occasional momentum of the oar propelling the boat is changed into 

 the more continuous action of the paddle. It is still a desideratum 

 in steam navigation to obtain a paddle wheel which shall expend all 

 its power most advantageously. In the ordinary paddle wheel, with 

 fixed float boards, the loss of power is very great, the float board as it 

 descends striking the water at an oblique angle and tending to raise 

 the boat out of the water rather than drive it onward j as it passes 

 through the water, it changes this oblique position for a vertical one, 

 and it is at this point only it is using its power effectively, for where 

 it commences to rise out of the water again, it is at another oblique 

 angle, lifting a body of water up before it, and tending to depress the 

 boat into the water. The float board which rises out of the water, has 

 the opposite efiect of the paddle which is entering the water, they 

 counteract each other, and hence much of the power of both is lost. 

 In the little model of the propelling wheel, which is upon the table, 

 much of this objection is overcome. The float boards in it are move- 

 able, and have a tendency always to retain the perpendicular in their 

 revolution, thus entering the water with httle resistance, and rising 

 out of it without creating much back-water : this is accomplished by 

 the off side of the paddle wheel having an additional false side at- 

 tached to its inner surface, and into which the pins of the float board 



