52 



Examples of 

 flow aftions 

 with ftrong 

 coarfe ferews* 



Cbfervation on 

 new or unufual 

 conftrudlions. 



Confidcrations 

 refpefting the 

 ufe of a double 

 fcrew in n\ea- 

 fariflg. 



Pump with a 

 double plunger 

 fcjr which the 



tHllEE ^fiCftANICAL STRXJCTURfiS. 



iht cotifequence is the fame as if a fingle fcrew had been ufe^, 

 having the diftance between thread and thread equal to the 

 difference beween the meafures of one thread in each. 



For example. Siippofe it were requiredto operate by ftrong 

 preflure with a fcrew of ten threads in the inch, it is evident 

 that the metallic protuberance or helix could not be quite fa 

 thick as one twentieth of an inch, and could not therefore 

 withftand any confiderable force. But by the prefent con- 

 ftru6lion we may make the thread as ftrong as we pleafe. Thus 

 if the flighteft be one quarter of an inch, the fineft fcrew muft 

 have two turns in the inch, and the coarfeft two turns in one 

 inch and one tenth ; or in other words, it would pafs over the 

 fame fpace in ten turns as the other fcrew would in eleven. 



It is evident that by the conftru6lion in the figure, the moft 

 powerful prefTure need not be ufed till the fubjed under com- 

 preflion has already been urged by thfe limple fcrew, 



I may here remark concerning every one of the contrivances 

 in this paper, that I have never confidered it as a requifite in 

 offering a machine or inftrument to the attention of my readers 

 that it fliould be univerfally better than the means already in ufe. 

 If it be new or unheeded, and ingenious, or capable of adding 

 to the flock of refources which intelligent conftrudors find fo 

 various in their degrees of eligibility under different circum- 

 ftances, it will, I am confident, be confidered with intereft, 

 and accepted with welcome. 



When we confider the extreme precifion of a limple fcrew, 

 if commonly well made, it may feem fcarcely neceffary to ufe 

 this contrivance in the meafurement of fmall quantities. The 

 fineft fcrews ufually to be met with do not exceed 100 threads 

 in the inch, and I do not know of any having been made finer 

 than of 200 threads in that fpace. Let us confider by what 

 threads of uiual or even coarfe number we could have a dif- 

 ference pet turn of above one thoufandth of an inch, and we 

 fhall find that the difference between a thread of 32 and one of 

 33 in the inch is the 1056 part of an inch. So likewife the- 

 difference between the 60th and 6lil of an inch is the 3660th 

 part, &c. I have a level trier on this conftru6lion which gives 

 the four hundredth of an inch for each turn of its compound 

 fcrew. 



Fig. II. Plate 4. is a pump defcribed to me by Mr. Bramah^ 



and I think his own invention. Between the collar of leathers 



4. A, 



