Fortin’s plan of constructing a barometer cistern is shown in fig. 2. The cistern is formed of a glass cylinder, which allows of the level of the mercury within being seen. The bottom of the cylinder is made of sheep-skin or leather, like a bag, so as to allow of being pushed up or lowered by means of a screw, D B, worked from beneath. This screw moves through the bottom of a brass cylinder, C C, which is fixed outside, and protects the glass cylinder containing the mercury. At the top of the interior of the cistern is fixed a small piece of ivory, A, the point of which exactly coincides with the zero of the scale. This screw and moveable cistern-bottom serve also to render the barometer portable, by confining the mercury in the tube, and preventing its coming into the cistern, which is thus made too small to receive it.