2 L. CASELLA'S CATALOGUE 



be easily seen. First, hang the board on the wall, then insert the lower part of the cistern 

 through the bottom bracket, and suspend the instrument as in fig. 1, p. 3. When the barometer 

 is thus suspended, unturn the thumb-screw till the mercury falls in the cistern to the level 

 of the ivory point 



To Set the Barometer. First read the attached thermometer, then adjust the mercury, 

 by means of the thumb-screw, so that it barely touches the ivory point in the cistern, which, 

 with its reflection will then appear as a 'double cone ; the height of the column is then 

 taken by adjusting the lower edge of the vernier, so that it shall exactly form a tangent to 

 the convex surface of the mercury in the tube, just excluding the light by keeping the eye in 

 the same plane with the back and front lower edges of the vernier. Every care should 

 also be used to avoid influencing the temperature whilst making the observation. 



How to Read the Vernier (fig. 1**, p. 4). By means of the annexed diagram, the use of 

 the vernier in ensuring accurate measurement is readily understood, c d represents part of 

 the fixed scale of the barometer, and a & is the sliding scale, or vernier. The scale c d is 

 divided into inches, tenths and half -tenths of an inch, so that each division of the scale is 

 *05 a & is made equal to 24 divisions of the scale, and is divided into 25 equal parts. It follows, 

 therefore, that each division of the vernier is smaller than each division of the scale, by the 

 25th part of '05 ; which is "002 inch. The lower edge of the vernier, a, is set to the top of 

 the barometrical column, and hence we have to find the height of a. First, we read on the 

 scale 29'15 ; next, we look along the vernier until we find one of its lines which lies evenly 

 with a line of the scale. As shown in the figure, this line is the second above 3. Now, 

 each of the figures engraved on the vernier count as hundredths, and each intermediate 

 division as two thousandths ('002) ; hence the vernier shows '034, and this added to the 

 scale reading 2915, gives the reading sought 29184. 



4. STANDABD BAEOMETEE, to revolve on cast-iron pedestal, as designed for the 

 Committee of the Royal Kew Observatory, and most of the leading Foreign 

 Observatories ; size of tube "08 in. internal diameter (fig. 4, p. 3) 24 



5. Standard Barometer for observatories, with extra large column of mercury, in neat 

 skeleton iron frame, arranged to revolve in brackets from the wall, or on 

 pedestal, precisely as the Kew standard, for reading off by means of the 

 cathetometer ...... 18 10 



6. CATHETOMETEE, large size, precisely as used at the Kew Observatory 



(jig. 6, p. 4) . . . . 18 and 21 



7. Standard Barometer^ on the Kew principle, in which the graduations of the 



scale are arranged to compensate for the rise and fall of mercury in the 

 cistern, by which the necessity of reading from a point in the cistern is 

 obviated. The mounting, etc., the same in every respect as No. I 

 standard barometer . . . * r 8 10 



8. STANDAED BAEOMETEE, as No. 7, in plainer mounting , t 600 



9. THE STUDENT'S STANDAED BAEOMETEE, on the Kew principle (as No. 7), with 



similar compensation, etc., but smaller in size, for those who do not at first 

 desire a more expensive standard . . . . 4 15 



10. STANDAED BAEOMETEE, on the Kew principle as No. 8, but with handsome bold 

 ivory or metal scale, with plain and broad graduations for easy reading, re- 

 volving in brackets on oak or mahogany board . . 600 



