278 PHILOSOPHICAL TKANSACTIONS. [aNNO 1751. 



" SIR, 



" I have now had thorough trial of the clock ; and am perfectly satisfied that 

 your pendulum takes oft' the effect of heat and cold, as well as either the gridiron- 

 pendulum (as it is commonly called) or the quicksilver pendulum ; and this upon 

 •sufficient trial for near 2 years. It has this advantage of both the fore-mentioned 

 ones, that it may, by lengthening or shortening the levers, be easily adjusted to 

 the exact proportion of the difference of the iron and brass, which neither of 

 those kinds is capable of, without very great trouble and difficulty. I was in- 

 deed prejudiced against the method of doing it by levers, as I had heard the late 

 Mr. Graham say, that he had tried levers in different ways, that he found they 

 did not work regularly and freely, but by jerks. However, in your method I 

 am satisfied, by the fullest experience, that they succeed as well as either of the 

 other sorts, or perhaps any other kind that may be invented hereafter," 



Before concluding, he observes that, in the year 1 748, he made a model of a 

 contrivance to be added to a pocket-watch, founded on the same principles, and 

 intended to answer the like purpose, as the pendulum above described. And, 

 at a meeting of a council of the Society, on Feb. 15th last, he produced a watch 

 which he had made for a gentleman, with this contrivance added to it, and also 

 the model, by which was shown what effect a small degree of heat would have 

 upon it. 



LXXXTI. Of a Neiv Tackle or Combination of Pulleys. By Mr. John 



Smeaton, p. 404. 



The axis in peritrochio, and the compound pulley, are the only mechanic 

 powers which can with convenience be applied for moving large weights, when 

 the height to which they are intended to be raised is considerable. The ex- 

 cellence of the former is, their working with little friction ; that of the latter, in 

 their being easy to be moved from place to place, and applied extempore, as oc- 

 casion requires. 



The present method of arranging pulleys in their blocks may be reduced to 2. 

 The first consists in placing them one by the side of another on the same pin ; 

 the other in placing them directly under one another, on separate pins. But in 

 each of these methods an inconvenience arises, if more than 3 pulleys are framed 

 in one block. For, according to the first method, as the last line, by wliich 

 the draught is made (or, as it is commonly called, tlie fall of the tackle) must 

 necessarily be on the outside pulley or shieve ; the difference of their friction 

 will give it so great a tendency to pull the block away, that as much will be lost 

 by the rubbing of the shieves against the block, on account of its obliquity, as 

 will be got by increasing the number of lines. 



The 2d method is free from this objection ; but, as the length of the two 



