Page:Transactions of the Provincial Medical and Surgical Association, volume 4.djvu/204

 102 is necessary to premise the conditions of the instruments, the corrections necessary for each, and the circumstances of the localities in which they are placed.

In the standard barometer of the Royal Society the tube is 33¾ inches long; its exterior diameter 0.86 inch, and the diameter of the bore 0.530 inch. The cistern is turned in well-seasoned mahogany, and there is a small cavity in its bottom to receive the end of the tube, which rests upon it; a groove communicates with the cavity, to ensure the free passage of the mercury. Everything has been studied in this instrument to render accuracy attainable, with as little trouble as possible to the observer. The diameter of the tube renders the correction for capillary action almost unnecessary. The correction for the capacity of the cistern has been contrived to be 1-100th of the result above or below the neutral point 30.576.

The barometer I have used at Great Malvern, is an instrument carefully constructed by myself. The tube is 34 inches long, its exterior diameter .5 inch, and the diameter of the bore .250 inch. The mercury has been boiled throughout in the tube. The cistern is turned in mahogany, and through the bottom the tube passes into a leathern bag firmly fixed to its circumference; a small hole forms a communication between the two. At one part of the circumference of the cistern an ivory point is fixed, and a screw, acting upon the lower part of the bag, will always keep the surface of the mercury just touching this point: thus the correction for the capacity of the cistern is rendered unnecessary.