M.Zeal London H M Govt, Floating Bath Thermometer British Made but when, in glass and wood, other materials and why?

M.Zeal London H M Govt, Floating Bath Thermometer British Made but when, in glass and wood, other materials and why? Why would the UK Government make a theromometer to float in the bath? Surely a duck would do?

Can anyone give a date? And its intended purpose.