Who introduced the hourglass profile of the tower ?
orlogiis vitreis " ) , price of each 4½ gross ' , in sterling 9 " s . " Item , For four horologes of the same sort ( " de eadem secta " ) , bought there , price of each five gross ' , making in sterling 3 " s . " 4 " d . "" Hourglasses were very popular on board ships , as they were the most dependable measurement of time while at sea .