Self-compensating Standard Barometer

This barometer has been suggested to Messrs. Negretti and Zambra by Wentworth Erk, Esq. It consists of a regular barometer; but attached to the vernier is a double rack worked with one pinion, so that in setting or adjusting the vernier in one position, the second rack moves in directly the opposite direction, carrying along with it a plug or plunger the exact size of the internal diameter of the tube dipping in the cistern, so that whatever the displacement that has taken place in the

istern, owing to the rise or fall of the mercury, it is exactly compensated by the plug being more or less immersed in the mercury, so that no capacity correction is required.

A barometer on this principle is, however, no novelty, for at the Royal Society’s room a very old instrument may be seen reading somewhat after the same manner.

Fig. 6 is an illustration of the appearance of this instrument. The cistern is so constructed that the greatest amount of light is admitted to the surface of the mercury.