PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 317 



manner as tlie sharp tliroad. The width of tlie flat in the bottom of thread 

 being- dependent upon the amount taken oflF the point of the tool, it becomes 

 necessary not only to determine what amount this shall be, but also pro- 

 vide a «ueans of measuring it. The angle of the proposed thread is fixed 

 at 60 deg\, the same as the sharp thread, it being more readily obtained 

 than 55 deg. and more in accordance with the general practice of the coun- 

 try. Divide the pitch, or, which is the same thing, the side of the thread, 

 into eight equal parts, take off one part from the top and fill in one part in 

 the bottom of the thread; then the flat top and bottom will equal one-eighth 

 of the pitch, and the wearing surface will be three-quarters of the pitch. 

 These i)roportions will give the depth of the thread almost precisely the 

 same as the English; and as the wearing surface on all screws will be con- 

 fined practically to the flat sides, we shall see that upon the proposed plaa 

 this will be 3G per cent, greater than on the English." 



Mr. Seller's article contains all the formulas and diagrams essential to a 

 complete comprehension of the subject. He also proposes a system of uni- 

 form dimensions for bolt-heads and nuts, ar:d gives the formula and tables 

 requisite to secure this desirable end. 



The proposition of Mr. Sellers seemed to meet the cordial approval of the 

 members generally. 



Mr. J. Wyatt Reid said some idea of the evil now sought to be avoided 

 might be had, Avhen he stated that at his establishment there were used six 

 difierent taps for the same sized bolts. 



Natural Weather Indicator. 



Mr. L. B. Page exhibited a very sensitive hygrometer, made by connect- 

 ing a rotating index hand with a curious and beautiful exotic, which grows 

 in the Desert of Arabia, and is called by the Arab the talisman or prognos- 

 ticator. A mercurial thermometer is attached to the hygrometer, and the 

 whole does not exceed ten inches in length. The plant used is very sus- 

 ceptible to weather changes, and coils or uncoils according to the dampness 

 of the atmosphere. It was found that the moisture in the human breath 

 was sufficient to give instantaneous motion to the index. 



The Chairman remarked that the Association have not tested the accuracy 

 of this instrument, but, from the number of certificates shown from scien- 

 tific gentlemen, there seems to be no question as to its practical value ia 

 the hands of the farmer or gardener. Prof. Henry, of the Smithsonian In- 

 stitute, says, in a letter written several years since: "It appears to be 

 peculiarly sensitive, and gives a greater range of motion than either the 

 animated oat or ordinai-y catgut." The wet bulb and hair hygrometers are 

 too delicate instruments for general use. This little instrument requires 

 no attention after it has once been properly adjusted; and, as it is said to 

 indicate a change in the weather several hours before the change will take 

 place, the farmer will have less difficulty in anticipating rain or sunshine 

 than with the ordinary barometer. 



Tlie following interesting items of scientific progress were presented by 

 the Chairman: 



