=39. Newman's Mountain Barometer.=--Fig. 29 is an illustration of the mountain barometer known as Newman's. The cistern consists of two separate compartments;--the top of the lower and the bottom of the upper, being perfectly flat, are pivoted closely together at the centres, so that the lower can move through a small arc, when turned by the hand. This movement is limited by two stops. The top of the lower compartment and the bottom of the upper have each a circular hole, through which the mercury communicates. When the instrument is required for observation, the cistern is turned close up to the stop marked "_open_" or "_not portable_." When it is necessary to pack it for travelling, the mercurial column must be allowed to fill the tube by sloping the barometer gently; then invert it, and move the cistern to the stop marked "_shut_" or "_portable_." In this condition, the upper compartment is completely filled with mercury, and consequently that in the tube cannot move about, so as to admit air or endanger the tube. Nor can the mercury pass back to the lower compartment, as the holes are not now coincident, and the contact is made too perfect to allow the mercury to creep between the surfaces. The tube does not enter the lower compartment, which is completely full of mercury when the instrument is arranged for observation. The spare capacity of the upper cistern is sufficient to receive the mercury which descends from the tube to the limit of the engraved scale, which in these barometers generally extends only to about 20 inches. A lower limit could of course be given by increasing the size of the cisterns, which it is not advisable to do unless for a special purpose. This barometer may be had mounted in wood, or in brass frame. If in wood, it has a brass shield, which slides round the scale part of the frame, so as to be easily brought in front of the tube and scale as a protection in travelling; the vernier screw, in this case, being placed at the top of the instrument. When the scale is graduated with true inches, the neutral point, the capacity and capillarity corrections should be marked on the frame. The graduated scales, however, placed on these barometers in brass frames, are usually artificial inches, like the Kew plan of graduation; the advantage being that one simple correction only is required, viz. one for index error and capillarity combined, which can always be readily determined by comparison with a standard barometer; moreover, as no adjustment of cistern is required in reading, the instrument can be verified by artificial pressure throughout the scale, by the plan practised at Kew, Liverpool, &c., and already described (see p. 18). 40. NEGRETTI & ZAMBRA'S PATENT MOUNTAIN AND OTHER BAROMETERS.