384! SUttVEY OP THE COAST 



glass of a long focus and large aperture has completely a 

 fancy price : — But it would be too tedious for me to enter 

 here into details on this subject. 



It may be proper for me to mention a source of error 

 which may affect the accuracy of the spirit levels. These 

 may generally be supposed to be adjusted at a mean tempe- 

 rature, of course a mean length of the bubble. At the iiigher 

 and lower temperatures in whicli it is often necessary to ob- 

 serve, the longer or shorter bubble may measure differently 

 in the course of the level, if this is not very regular ; besides 

 that its oscillation will at all times become sluggish and more 

 irregular. 



The manner of packing the instruments is of considerable 

 importance for the preservation of their accuracy. The 

 boxes ought always to be of light and straight grained wood. 

 No piece must ever be fixed in a direction diagonal to the 

 grain, as it will be pressed out of shape by the drying of the 

 wood. Any piece fastened separately must always be placed 

 in the direction of the length of the fibres. In travelling, 

 the instrument should always be placed as much as possible 

 in the same position in which it is used, those parts only 

 which are not fixed to it being taken off and packed sepa- 

 rately. The centre being the part which bears the greatest 

 weight, must be the most firmly supported. Any stay used 

 to steady the instrument in the box must be placed against 

 one of its solid parts. No circle nor telescope should ever 

 be suffered to toucii the box, nor have any bearing to sup- 

 port or steady the instrument. They must be suspended 

 freely in the box, by the parts destined to hold them. It 

 is even advisable, in all cases in which the weight of the in- 

 strument allows this precaution, to make the boxes as light 

 as possible, that in case of accident, they may break, belbre 

 ati effectual reaction upon the instrument can take place. 



The larger instruments of the collection for the survey of 

 the coast all travel in their boxes in the position used in ob- 

 servation, and arc supported according to the aliove princi- 

 pies ; and Mr. Troughton agreed with me in preferring this 



