206 Instructions for the Scioitific Expedition 



at one or more temperatures (the wider asunder the better), marked 

 upon their scales and applied as index errors. This is particularly 

 necessary with the register thermometers, whose construction ren- 

 ders them most liable to such errors. 



In placing the standard thermometer, an exposure should be 

 chosen perfectly shaded from the sun, one where no reflected sun- 

 beams from water, buildings, rocks, or dry soil can reach it, and one 

 which is easily accessible for observation. It should be fixed, not 

 merely hung, upon a bracket projecting six inches from the wall, or 

 other support to which it may be attached, and it must be completely 

 sheltered from rain by a screen, so that the bulb shall never be wetted. 

 In reading it, the observer should avoid touching, breathing on, or 

 in any way warming it by near approach of his person ; and in night 

 observations particular care should be taken not to heat it by ap- 

 proximation of the light. The quicker the reading is done the 

 better. 



Notice should, of course, be taken of all sudden and remarkable 

 changes of temperature, although such occasional observations must 

 not be recorded in the regular series. 



The self-registering thermometers should be placed with the same 

 precautions as the standard, and so fastened as to allow of one end 

 being detached, and lifted up to allow of the indices within the 

 tubes sliding down to the ends of the fluid columns, which they will 

 readily do with the assistance of occasional tapping. 



The self-registering thermometers are apt to get out of order by 

 the indices becoming entangled, or from the breaking of the column 

 of fluid. When this happens with the spirit thermometer, it may 

 be rectified with ease by jerking the index down to the junction of 

 the bulb and tube. The whole of the tube will at the same time 

 become wetted with the spirit, and by setting it on end with the 

 bulb downwards the spirit will run together into one continuous co- 

 lumn. 



When the steel index of the mercurial thermometer becomes im- 

 mersed in the mercury, it must be jerked in the opposite direction, 

 till it, with the mercury which may be above it, is projected into 

 the little bulb at the top of the tube. If this do not succeed, heat 

 must be applied to the mercury-bulb, and when the index is fairly 

 lodged in the air-bulb, by carefully warming the mercury-bulb with 

 a spirit lamp having a very small flame, the mercury must be made 

 to expand till it rises to the very top of the tube, and projects 

 convexly into the air-bulb. The tube must then be placed up- 

 right, and, by tapping, the detached mercury will slip down beneath 

 the steel index, and will fairly unite with the convex projection 

 aforesaid. Now let the bulb cool, and the mercury will sink in one 

 united column, and leave the index free. 



Besides the regular series of observations of the temperature of 

 the air, there are other occasional observations to be made of tem- 

 perature under different circumstances, which might possess great 

 interest. 



