ts aperture by the depression of the diaphragm upon the end of a pipe by means of a screw-plunger.
A plate beneath the stage of a compound microscope, to restrict the amount of light reflected from the mirror.
The plate has a number of holes of varying sizes, either of which may be brought to bear.
A pump in which a diskpiston is attached by an elastic diaphragm, usually of leather, to the sides of the barrel.
It was described by Desaguliers in 1744 as a piston without friction.
It is much older than the time of this philosopher, however.
It has been again and again re-invented, and brought out with a flourish of trumpets.
See bag-pump. Its application may have been suggested by the human diaphragm.
A philosophical instrument for measuring distances.
（Architecture.) A system of columniation in which the width of the intercolumns is equal to three diameters of a column.
lleable metal, is conducted in a crucible covered with small charcoal and placed in a hearth similar to the foregoing.
（Optics.) An instrument for measuring altitudes and angular distances, invented by Mayer about 1744, and afterward improved by Borda and Troughton.
In principle and construction it is similar to the sextant (which see), the graduations, however, being continued completely round the limb of the circle.
Troughton's has three arms radiating fro coincidence with the other seen by direct vision; the apparent angular distance is thus doubled, and the graduations of the circle, which are continued all the way around its limb, thus embrace 720° of are.
It was first invented by Mayer about 1744.
Borda, after whom it is frequently called, in 1780, introduced a second arm and vernier, to which Troughton subsequently added a third.
The mean of the readings given by each arm is taken as the true angular measurement, which may be repeated
might take care of itself, and even these regulations were evaded and disregarded.
The streets were subsequently lighted by contract for 117 nights in the year.
This proving unsatisfactory, an act of Parliament was passed in 1736, which authorized the corporation to establish glass lamps to be lighted from sunset to sunrise all the year round, and the number was increased to 5,000 within the bounds of the city proper; probably 15,000 within the group of municipalities known as London.
In 1744, thelight-ing of the city was still farther regulated by act of Parliament, and at a somewhat later period Oxford Street alone was said to contain more lamps than all Paris.
Misson, the celebrated traveler, who visited England about 1680, states that in the streets of London, instead of lanterns, they set up lamps which, by means of a very thick convex glass, throw out great rays of light which illuminate the path for people that go on foot tolerably well.
These lamps were at every tenth