8 STOKES.— PENDULUM OBSERVATIONS. 



At the time of equinox <r=0, and hence the 48-hour observation, 

 if made at this time, and also when />c=0, would show the non- 

 existence of a diurnal tide, although there might be really a large 

 one. The form of equation (2.) shows the reason for directing the 

 observations to be made when the Moon's declination is great. 



As a rule the diurnal tide is of considerable amount both lunar 

 and solar, in all the branches of the Davis' Strait tidal wave ; and 

 in some cases the solar diurnal tide is actually greater than the 

 lunar diurnal tide. 



in. GrENERAL RuLES FOR TiDAL OBSERVATIONS. 



Much valuable time has been often misspent on tidal observa- 

 tions of little value, and great disappointment felt at the small 

 results produced by most laborious and carefully conducted 

 observations ; whereas at other stations, a simple month's obser- 

 vations properly made have given results of great value, although 

 the observations themselves did not cost one-tenth part of the 

 labour of other observations which gave but little result. 



I offer the following suggestions for tidal observations m.ade for 

 a lengthened period. 



1. Hourly observations of height should be made for one month 

 at the times of solstice and equinox. 



2. At the intervening periods, in order to save the labour of 

 the observers as much as possible, it is recommended (instead of 

 noting the time and height of high and low water each day) that 

 the height of the tide should be registered every four hours 

 of mean solar time. This would correspond with the times of 

 striking bells, which would ensure punctuality and accuracy as to 

 the time of observation, and the observation itself could be made 

 in one minute. I should prefer observations made every four 

 hours, for this reason among others, that the diurnal and semi- 

 diurnal tides could be at once separated, and discussed indepen- 

 dently of each other. 



3. The times of observation must be carefully kept to, but 

 whether the exact hours, or a fixed number of minutes after the 

 exact hours, may be decided according to the convenience of the 

 observers. 



4. Remark carefully that the times of observation must be ac- 

 cording to mean solar time, not according to apparent solar time. 



3. Pendulum Observations. By Prof. Stokes, M.A., 

 Sec. RS. 



It must be remembered that pendulum observations are of little 

 value unless very accurately made. 



The pendulum station will of course be adjacent to the ship's 

 winter quarters. It must if possible be on land, chiefly because 

 the clock's rate at the time of observation must be determined by 

 transits, and we have no guarantee that ice covering the sea, how- 



