218 SCIENTIFIC APPARATUS. 



These barometers must always be tested in a vacuometer, so 

 that the pressure can be artificially increased and diminished. 

 The vacuometer contains a standard barometer, and the two 

 instruments are then read by means of the cathetometer under 

 various pressures, ranging from 27 inches to 31 inches, and the 

 error at each half inch is noted. This error, of course, con- 

 tains the correction for capacity as well as of graduation of the 

 scale, &c., &c. 



Marine barometers have one peculiarity in construction which 

 is not apparent, but which renders them less immediately sensitive 

 to sudden changes of pressure than instruments intended for land 

 service. The motion of the ship would cause the column of mer- 

 cury in an ordinary tube to oscillate up and down in such a way, 

 that at times an accurate reading could not be taken. This 

 motion is called " pumping," and to obviate it, a part of the tube 

 is made of a very fine bore, so that the upper portion, which is of 

 the full diameter, is very short, and there is less mercury exposed 

 to the pumping action. This modification meets the purpose for 

 which it was designed, and, as the instruments are practically 

 found to be not too sluggish for use on land, the Kew construction 

 is being gradually more and more commonly used for all purposes, 

 the facility of transport and of reading, as compared with a Fortin, 

 being very great points in its favour. 



Kew barometers are exhibited by Adie. 



B. Syphon Barometers. This method of construction is much 

 used on the Continent, especially for Standard Barometers. It 

 was first devised by Boyle, improved by De Luc and by Gay- 

 Lussac. It is much used for mountain barometers. The new 

 syphon barometers used in Russia are exhibited. 



These instruments must be provided with a double scale, as the 

 surface of the mercury must be observed in each leg. The fact 

 that these two readings are taken in the same way, which is not 

 the case in Fortin's instrument, is the great reason of the approval 

 of the pattern for standard barometers. 



