322 The adaptation of Aneroid for surveying in India. No. 4. 



Under all these circumstances were the improvements of which the 

 Aneroid is susceptible carried into effect, it is, taking it altogether, one 

 of the most convenient instruments of which the traveller can make 

 use within the limits to which it is trustworthy, whatever these may be. 

 The following description of the Aneroid taken from Dr. Purdie 

 (Thompson's Manual) will make what is about to be stated more clear 

 than it otherwise might have been. 



length of 30° at these temperatures. But it is evident that this length would be 

 greater than 30° if we drive out a portion of the mercury, to make 212° stand at the 

 point where 62° stood when the scales were measured. This is corrected by care- 

 fully weighing the tubes before and after regulating them for 212°, and the propor- 

 tion is stated : if the larger quantity of mercury give the length noted, the dimi- 

 nished quantity of mercury from regulation to 212° will give a diminished scale, 

 which scale is the true or corrected one, to be divided on the thermometer ; each 

 degree is subdivided into fifth or tenth parts and cut on the glass stem of the ther- 

 mometer ; or may be laid down on an attached scale. 



When the thermometer is to be used, the bulb must be carefully inspected to 

 see that there are no small detached globules of air attached to the interior of the 

 bulb : should such be found they are to be removed by shaking in a larger globule 

 from the contracted part of the bulb, and making it pass over the smaller globules, 

 which it will take with it ; it is then to be returned to the contracted part ; and 

 should any small portion of the mercury lodge in the tube, it is to be joined to the 

 column by heating the bulb till it rise to the small bulb at the top of the thermo- 

 meter, where the detached portions will unite. 



The best method of using these Thermometers is to have the bulb and column of 

 mercury up to the reading point brought to the boiling temperature : this is best 

 done by a boiler provided with telescope slide-tubes, which can be regulated to any 

 required length ; or where such an apparatus is not at hand, the same length of 

 column, as nearly as possible, should be kept out of the water. Professor J. D. 

 Forbes (Philosophical Transactions, Edinburgh, Vol. XV, page 409) has with great 

 care determined the difference of altitude due to a change of 1° in the boiling point 

 of water, and found it to be 549. 5° for each degree of Fahrenheit. Thermometers 

 used for this purpose should be frequently compared one with another, and then- 

 differences noted ; or where one only is used, the instrument should be noted as 

 frequently as possible, both for the purpose of obtaining more perfect results from 

 a mean of the observations, and for correcting small changes in the indication 

 which go on in course of time. 



For security in carriage, the Thermometer is enclosed in a brass case and sup- 

 ported at all points by woollen stuffing, and is removed from its case by screwing 

 off the top and bottom, and pushing out the bulb when the Tliermo meter may be 

 drawn out." 



