340 



HYDROGRAPHICAL SURVEYING [chap. xiv. 



Vibra- 

 tions and 

 Shocks. 



It should be excessively small, and our opinion is that in the 

 practical question of meridian distances the labour of ascer- 

 taining it is not repaid by the result. It is difficult to 

 separate the error due to this from that originating in 

 defective mechanism, and though formulae have been elabor- 

 ated for its detection, we do not propose to give them 

 here. 



4. Vibrations and Shocks. — However well chronometers may 

 be stowed, the jars from seas striking the ship, and other like 

 accidents, must be communicated more or less to the chrono- 

 meters. The vibration of the screw is in some vessels sufficient 

 to pass tlirough all the soft cushions in which they may lie, and 

 must have its effect, more especially from the fact that the 

 watches themselves are hanging in the metal gimbals, in which 

 there must be play sufficient to allow them to swing easily, 

 and therefore enough to set up small shocks on any violent 

 movements of the ship.* 



* In connection with the observations of the transit of Venus of 1874, 

 Lord Lindsay conveyed nearly sixty chronometers to Mauritius. These 

 were kindly permitted by him to be used in assisting to determine the 

 meridian distance between Mauritius and Eodi-iguez, when they were 

 shipped on board H.M.S. Shearwater, under the author's command. As 

 the results by these watches, both of the distance between Mauritius and 

 Rodriguez, and Mauritius and Aden (between which latter places they were 

 conveyed in the mail steamer), were remarkably good, and as the results by 

 the Shearwater s chronometers which were admitted into the distance 

 Mauritius to Rodriguez were not so satisfactory, a description of the manner 

 in which Lord Lindsay's watches v.-cre stowed may not be out of place. We 

 may add that the Shearwater had to beat up for eight days against a strong 

 trade-wind on one occasion, and was a very lively ship. 



The watches were taken out of their gimbals and placed in square boxes, 

 which held nine of them each. The partitions of theso boxes were thickly 

 stuffed with very soft material (cotton- wool) covered with satin, so that each 

 watch lay in a bed of down which was made exactly to tit it. 



Each box was fitted with a metal framework after the fashion of gimbals, 

 the outer pivots of which fitted into carefully turned sockets, in two upright 

 columns of wood, which were firmly screwed to the deck. Each pair of 

 uprights carried three boxes of watches. 



The effect of this was that any slight shocks to the boxes caused by seas 

 striking the ship, or by longitudinal slipping of the pivots, were entirely 

 deadened before reaching the watches themselves. 



This mode of stowing necessitated taking the watch up bodily in the hand 

 to wind, which at first sight seems dangerous, and undoubtedly does present 

 more opportunity for accident than the ordinary method ; but, as far as the 

 author is aware, none took place during the five or six months the watches 

 were thus treated, and the admirable agreement of the results seems to show 

 that this system was unusually successful. 



Whether it could be adopted on board men-of-war, especially small ones, 

 which are usually employed in surveying duties, ia another matter, as it 



