220 SCIENTIFIC APPARATUS. 



form and to the same time-scale the various meteorological 

 elements and the height of the tide. 



The third method also gives intermittent records. The simplest 

 form of it is the barograph known as Sir A. Milne's, in which the 

 record is produced by a pin attached to a float on the surface of 

 the mercury in the open limb of a syphon barometer. This pin 

 is driven into paper by clock-work at regular intervals. 



This principle was first employed by Alexander Gumming in 

 1766, and the original instrument, called a " Clock Barometer," 

 came into Mr. Luke Howard's possession, and was used by him 

 from the year 1815. 



Aneroids and Metallic Barometers are instruments very gene- 

 rally used as weather-glasses, but they cannot lay claim to be con- 

 sidered scientific instruments of the same character as mechanical 

 barometers. Their principle was first devised by Vidi, and is that 

 the pressure is measured by means of a volume of air which is 

 confined in a box or case, partially exhausted and hermetically 

 sealed. According as the pressure of the atmosphere increases or 

 diminishes, this enclosed air is compressed or the contrary, and 

 the envelope alters its shape accordingly. The motion resulting 

 from this alteration of shape can be transferred to a hand moving 

 on a dial, or can be made to record itself mechanically. 



Messrs. Goldschmid, of Zurich, exhibit several specimens 01 

 their aneroids. 



The objections to the aneroid for scientific observations arise 

 from the fact that its action depends on the accuracy and delicacy 

 of its construction, while its interior fittings, being metallic, are 

 liable to corrosion, &c. Moreover, its action is liable to alteration 

 owing to rough usage, as in travelling. The errors in the indica- 

 tions of aneroids can only be detected by comparison with the 

 metallic barometer, and such comparisons should therefore con- 

 stantly be carried out, when occasion offers, if the aneroid is to be 

 of any use. 



