In order to increase the extent of the baro metric changes, a contrivance is sometimes adopted, called the diagonal barometer, which is represented in Fig. 4. b c d is the glass tube bent at c, the altitude of which is less than 28 inches; hence c b includes the whole barometric range in the present form, while a c is the range it would have were the whole of the tube vertical. Now it is mani fest, that by decreasing the angle at c, so as to bring b c nearer the horizontal position, we can snake its proportionate length to a c as great as we please. Suppose 6 c so inclined that its length shall be three times greater than a c, then every rise or fall of 1 inch in a c would be equivalent to a rise or fall of 3 inches in b c. The difficulty, however, of observing the precise height of the mercury in this arrangement, more than counter balances the advantage resulting from the extended range, and this form is, therefore, seldom adopted.
The wheel barometer is another contrivance for enlarging the scale, and rendering minute changes more easily observed. This, which is the common domestic barometer, is represented in Fig. 5, in which d 6 is the longer leg, a b the shorter, in which the changes of altitude occur : for as the diameter of the bulb d at the top is very large, compared with the diameter of the tube at c, a small fall in d will be equivalent to a considerable rise in the tube a b. Thus the surface at c may rise 3 inches, and thereby shorten to that amount the distance between the two surfaces of the column, which is the true height sus tained by atmospheric pressure, without sensibly af fecting the level at d At c there is an iron ball, floating on the surface of the mercury, and partly supported by means of a thread passing over the pulley p, and carrying a small counterpoise at w. On the axis of the pulley, an index i is fastened, which moving with it, branches the circumference of the circular plate, on which a scale is drawn, to repre sent the rise or fall in inches ; and also the terms fair, change, rain, ife., which certain altitudes have been improperly considered to indicate. In this arrangement, it is manifest that as the column is supported by the air's pressure on the surface e, any diminution of that pressure will cause the mer cury in the longer leg to fall, and that in the shorter to rise. On the other hand, if the atmospheric pressure increases, the mercury will rise in the longer, and fall in the shorter, leg ; but, as before observed, the change of level will be scarcely per ceptible in the former, on account of the enlarged diameter of the upper part. The changes, then, that occur in the shorter leg, may, without material error, be considered the representatives of the changes that are continually occurring in the atmosphere. Now it will be evident, on inspection, that as the ball c is
partly supported by the mercury, it will partake of its motion. If the pressure of the air increase, the surface at c will be depressed, and as the iron ball must sink with it, the thread to which it is attached will, at the same time, communicate its motion to the pulley p, and through it, to the index, which will, consequently. move from the right towards the left of the graduated circle. On the other hand, if the atmospheric pressure decrease, the surface at c will rise, and, with the assistance of the counterpoise w force up the iron ball, and, by this means, turn the pulley and index in an opposite direction. There are two sources of error in this instrument, that render it inferior to the simple vertical barometer. These are, the pressure of the iron ball on the surface of the mercury, which necessarily increases the height of the shorter column, and the friction of the pulley. It is impossible entirely to annihilate these causes of error, and hence, for philosophical purposes, the straight barometer is preferred.
The Syphon Barometer is an instrument used to ascertain the pressure of air in the partially exhausted receiver of an air-pump. It consists of a tube bent as in the accompanying engrav ing, Fig. 6. Each leg may be about 4 inches in length, and one a b completely filled with puri fie d mercury. When the instrument is con nected with the air-pump by means of the screw at d, and the air partly exhausted, the mercury in the leg a b will begin to fall, and, conse quently, rise in the branch c b; every inch, therefore, that it falls in a b, must be reckoned equal to two inches in the straight barometer. This instrument does not begin to act till the air is reduced to about of its original density ; but this is no inconvenience, as its indications are seldom required till the exhaustion is nearly complete. If the leg a b be lengthened, and left open at the top, as in IV. 7, this barometer becomes a useful appendage to the steam engine, in ascertaining the pressure of steam within the boiler. When it is attached to the boiler by the screw d, and the air or steam in the boiler has the same elasticity as the external air, the mercury stands at the same height in both legs ; but as soon as the steam increases in elasticity, the mercury will be depressed in the leg c b, and rise in a b, and the difference between the two levels will be proportional to the difference between the external and internal pressure. If the common barometer stands at 30 inches, and the difference of level between the two surfaces is 6 inches, the elasticity of the steam will be or greater than the pressure or elasticity of the atmosphere.