THERMOMETER AND PYROMETER. 



38 



enced by atmospheric pressure, when 

 its cover is removed ; but the effects of 

 barometrical variations are scarcely 

 appreciable except in an air thermo- 

 meter ; and when made on a large 

 scale, is applicable, as Mr. Keith 

 has shown, to the important purpose of 

 marking the periods of the atmospheric 

 changes of temperature. This he effect- 

 ed by making the large tube 40 inches 

 long, but retaining the original width; 

 "Awhile the small tube is increased in 

 diameter, but not in length. The float c 

 is enlarged, and the float wire carries, 

 instead of the knee, a soft pencil, which 

 is made to press lightly against a hollow 

 vertical cylinder, 7 inches long and 5 

 in diameter, moved by clock-work, once 

 round in 31 days. This cylinder is 

 covered with smooth paper, ruled lon- 

 gitudinally into 31 columns, to corre- 

 spond to the days of the month ; and 

 every column is subdivided into 6 equal 

 parts, each corresponding to 4 hours. 

 The cylinder is ruled across into 100 

 divisions, intended to correspond to the 

 1 00 of Fahrenheit marked on the or- 

 dinary scale of the instrument, which is 

 unnecessary when the cylinder is applied. 

 Thus, as the cylinder revolves, the 

 point of the pencil will trace a line on 

 the paper more or less deviating from a 

 horizontal line, as the mercury rises and 

 falls ; and thus the paper will present a 

 chart of the variations of the thermo- 

 meter for a whole month, the value of 

 which, in degrees of Fahrenheit, will be 

 indicated by the numbers on the margin 

 of the paper. Keith recommends the 

 observer to have a copper plate for 

 giving ruled impressions on smooth 

 paper, to be applied monthly to the 

 cylinder ; and these, bound up together, 

 will present tabular views of the fluc- 

 tuations of the thermometer for everv 



month. It is hardly necessary to state 

 that a similar contrivance is applicable 

 to the indications of the barometer. 



6. We are indebted to Mr. Henry Home 

 Black adder, for some very ingenious 

 methods of ascertaining the tempera- 

 ture of the air, at any given hour, by a 

 subsequent inspection of a thermo- 

 meter. His first invention resembles 

 one of Rutherford's thermometers, sus- 

 pended on a pivot. If a spirit ther- 

 mometer be preferred, it is to be hung 

 vertically and inverted, so that the index 

 may rest on the last film of the liquid. 

 Suppose that we desire to know the 

 temperature at 5 o'clock, A. M., a lever 

 connected with a clock is applied, so as 

 to bring the thermometer to a horizontal 

 position at that hour ; and, at the same 

 time, the motion causes the bulb of the 

 thermometer to approach some 'source 

 of heat a little higher than that of the 

 air ; as, for instance, a small lamp, by 

 which the spirit would rise beyond the 

 now horizontal index, leaving it at the 

 point to which the spirit had contracted 

 before the reclination of the instru- 

 ment. When a mercurial thermometer 

 is employed, the instrument is also hung 

 vertically, but with its bulb lowermost ; 

 and the index, therefore, resting on the 

 mercury. It is brought to a horizontal 

 position by the same means as the other 

 thermometer ; and then its bulb, com- 

 ing into contact with a camel-hair pen- 

 cil, kept continually moist with water, 

 is cooled so as to cause the mercury to 

 shrink, and leave the index at the height 

 of the column while the instrument was 

 in the upright position. 



By a subsequent improvement he 

 has contrived to dispense with the index 

 altogether. This modification is seen in 

 Jig. 34, where two thermometers are 

 placed parallel on the same piece of box 



Fig. 34. 



wood or ivory : a 6 is a common mer- 

 curial thermometer ; c c? is of the same 

 size, but is not hermetically sealed. The 

 end of its stem is ground flat, and is in- 

 troduced into the neck of a small ball at 

 e ; which, as well as the stem, contains 



some mercury. The stem is pushed up 

 until it just reaches the ball, to which it 

 is cemented by colourless varnish. 



When these thermometers are in the 

 upright position, the globule of mercury 

 in e covers the orifice of the tube ; and 



